fix(#2046): config-set <key> null unsets (removes) the key instead of writing "null"

`gsd-tools config-set <key> null` — the "Clear" action documented in
settings-integrations.md / settings-advanced.md — previously fell through the
value parser as the literal STRING "null" and persisted it. Consequences:
"cleared" keys stayed set (config-get returned truthy "null"), and for secret
keys (brave_search/firecrawl/exa_search) a masked success line hid a truthy
4-char value on disk that integrations could pass along as a real credential.
There was also no unset/delete verb at all.

Fix: parse a bare `null` to JS null and short-circuit to a real UNSET that
DELETES the key from config.json — the semantic the docs already describe
("Remove the stored key" / "remove the key by setting it to null"). Deleting
(not persisting JSON null) is the correct clear: a persisted null is still a
present value consumers must special-case.

- src/config.cts:
  - parse block: `else if (val === 'null') parsedValue = null;`
  - cmdConfigSet: when parsedValue === null, short-circuit BEFORE the typed
    per-key validator gauntlet (so clearing an enum/boolean/number key removes
    it rather than being rejected) and before the project_code special-case;
    mask the previous value for secret keys in the output.
  - new `unsetConfigValue()` + `_unsetNestedValue()` mirroring setConfigValue/
    _setNestedValue: same prototype-pollution guard, but never creates missing
    intermediates and never prunes empty parents; returns { previousValue,
    existed }. Unsetting a never-set key is an idempotent no-op success.
- tests/config.test.cjs: new suite covering non-secret routing key, secret key,
  typed-enum-key bypass (context), idempotent unset, literal-"null"-on-disk
  guard, the unset-path prototype-pollution guard (alert #26 parity), and a
  4-segment deep-nested unset.
- tests/review-model-config.test.cjs: update the stale round-trip test that
  codified the bug (asserted config-set null → config-get returns "null") to the
  fixed contract — the model key is removed; the review workflow's
  `[ -n "$VAR" ] && [ "$VAR" != "null" ]` guard handles the empty read as
  "no override → reviewer default", same as the old "null" sentinel.

The 4 documented "Clear" flows (settings-integrations.md, settings-advanced.md)
were verified — their prose already describes removal, so the fix makes them
accurate rather than aspirational; no doc wording change required.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/config.cts

@@ -38,6 +38,14 @@ interface SetConfigValueResult {
   previousValue: unknown;
 }
 
+interface UnsetConfigValueResult {
+  updated: boolean;
+  unset: true;
+  key: string;
+  value: null;
+  previousValue: unknown;
+}
+
 interface WorkstreamContext {
   configPath?: string;
   [key: string]: unknown;
@@ -452,6 +460,87 @@ function _setNestedValue(
   return previousValue;
 }
 
+/**
+ * Deletes a value from the config object, allowing nested values via dot
+ * notation (e.g., "review.models.gemini"). Mirrors `_setNestedValue`'s
+ * prototype-pollution guard on every path segment (including intermediates).
+ *
+ * Unlike `_setNestedValue`, this NEVER creates missing intermediate objects —
+ * if any segment along the path is missing (or not a plain, non-array
+ * object), the key doesn't exist and we return early without mutating
+ * `config` at all.
+ *
+ * Does not prune now-empty parent objects after deletion (matches the
+ * conservative, structure-preserving behaviour callers expect from a bare
+ * unset).
+ *
+ * Returns { previousValue, existed } — existed is false when the leaf key
+ * (or an intermediate segment) was never present.
+ * Calls error() (process.exit(1)) on prototype-pollution attempts.
+ */
+function _unsetNestedValue(
+  config: Record<string, unknown>,
+  keyPath: string,
+): { previousValue: unknown; existed: boolean } {
+  const keys = keyPath.split('.');
+  let current: Record<string, unknown> = config;
+  for (let i = 0; i < keys.length - 1; i++) {
+    const key = keys[i];
+    if (key === '__proto__' || key === 'prototype' || key === 'constructor') {
+      error('Invalid config key (prototype pollution guard): ' + keyPath, ERROR_REASON.CONFIG_PARSE_FAILED);
+    }
+    const existingChild = current[key];
+    if (existingChild === undefined || existingChild === null || typeof existingChild !== 'object' || Array.isArray(existingChild)) {
+      // Path doesn't exist — nothing to unset, and we must not create it.
+      return { previousValue: undefined, existed: false };
+    }
+    current = existingChild as Record<string, unknown>;
+  }
+  const lastKey = keys[keys.length - 1];
+  if (lastKey === '__proto__' || lastKey === 'prototype' || lastKey === 'constructor') {
+    error('Invalid config key (prototype pollution guard): ' + keyPath, ERROR_REASON.CONFIG_PARSE_FAILED);
+  }
+  const existed = Object.prototype.hasOwnProperty.call(current, lastKey);
+  const previousValue = current[lastKey];
+  if (existed) {
+    delete current[lastKey];
+  }
+  return { previousValue, existed };
+}
+
+/**
+ * Deletes a key from the config file, allowing nested values via dot
+ * notation. Mirrors `setConfigValue`'s load/lock/write cycle.
+ *
+ * Does not call `output()`, so can be used as one step in a command without triggering `exit(0)` in
+ * the happy path. But note that `error()` will still `exit(1)` out of the process.
+ */
+function unsetConfigValue(cwd: string, keyPath: string): UnsetConfigValueResult {
+  const configPath = path.join(planningDir(cwd), 'config.json');
+
+  return withPlanningLock(cwd, () => {
+    // Load existing config or start with empty object
+    let config: Record<string, unknown> = {};
+    try {
+      if (fs.existsSync(configPath)) {
+        config = JSON.parse(fs.readFileSync(configPath, 'utf-8')) as Record<string, unknown>;
+      }
+    } catch (err) {
+      error('Failed to read config.json: ' + (err as Error).message, ERROR_REASON.CONFIG_PARSE_FAILED);
+    }
+
+    const { previousValue, existed } = _unsetNestedValue(config, keyPath);
+
+    // Write back
+    try {
+      platformWriteSync(configPath, JSON.stringify(config, null, 2));
+      return { updated: existed, unset: true, key: keyPath, value: null, previousValue };
+    } catch (err) {
+      error('Failed to write config.json: ' + (err as Error).message);
+    }
+  }) as UnsetConfigValueResult;
+}
+
 /**
  * Sets a value in the config file, allowing nested values via dot notation (e.g.,
  * "workflow.research").
@@ -586,6 +675,7 @@ function cmdConfigSet(cwd: string, keyPath: string | undefined, value: string |
   let parsedValue: unknown = val;
   if (val === 'true') parsedValue = true;
   else if (val === 'false') parsedValue = false;
+  else if (val === 'null') parsedValue = null;
   // #1581: Number.isFinite (not !isNaN) so 'Infinity'/'-Infinity' are NOT
   // coerced to non-finite numbers that JSON.stringify later renders as `null`
   // (disk=null while the CLI echoed 'Infinity'). They fall through to the
@@ -596,6 +686,25 @@ function cmdConfigSet(cwd: string, keyPath: string | undefined, value: string |
     try { parsedValue = JSON.parse(val); } catch { /* keep as string */ }
   }
 
+  // #2046: a bare `null` unsets (deletes) the key — the documented "Clear" action.
+  // Short-circuits before every typed per-key validator so clearing a typed key
+  // (enum/boolean/number) removes it rather than being rejected. Deleting (not
+  // persisting JSON null) is the correct "clear": a persisted null is still a
+  // present, truthy-adjacent value that consumers must special-case — worst for
+  // secret keys where a leftover value can be passed as a real credential.
+  if (parsedValue === null) {
+    const unsetResult = unsetConfigValue(cwd, kp);
+    if (isSecretKey(kp)) {
+      const maskedPrev = unsetResult.previousValue === undefined
+        ? undefined
+        : maskSecret(unsetResult.previousValue as Parameters<typeof maskSecret>[0]);
+      output({ ...unsetResult, value: null, previousValue: maskedPrev, masked: true }, raw, `${kp} unset`);
+      return;
+    }
+    output(unsetResult, raw, `${kp} unset`);
+    return;
+  }
+
   // #1581: project_code is an identifier string — never number-coerce it. A
   // leading-zero code like '007' must persist verbatim (not collapse to 7).
   if (kp === 'project_code') {