feat(#3274): make reviewer-lane timeout configurable via timeoutConfigKey (#4083)

* feat(#3274): make reviewer-lane timeout configurable via timeoutConfigKey

Add an optional `timeoutConfigKey` field to the reviewer lane descriptor,
resolved in `resolveLanePlan` at invocation time and falling back to the
frozen `timeoutFloorMs` when unset or invalid, in the same spirit as the
existing `promptBudgetKey`/`modelConfigKey` fields. All 12 shipped lanes
declare `review.timeouts.<slug>` on both surfaces (the descriptor and their
capability.json manifest), validated by capability-validator.cjs.

For the antigravity lane, the native `agy --print-timeout` flag — previously
a second hardcoded literal (`540s`) independent of the outer cap — is now
derived from the same resolved outer timeout in `antigravityArgv`, preserving
the existing 60-second buffer relationship (ADR-2782 D6: the outer bound is
declared data, the inner one is handler-owned).

The antigravity default timeoutFloorMs stays at 600s per the maintainer's
disposition; users raise it through the new config key instead.

* docs(#3274): document review.timeouts.* and extract resolveTimeoutMs helper

Address code-review findings on the timeoutConfigKey change: extract the
inline timeout-resolution logic into a named, exported, directly-tested
resolveTimeoutMs helper (matching the file's existing configString/
normalizeHost convention); document the new review.timeouts.* federated
config keys in docs/CONFIGURATION.md, docs/reference/capability-manifest.md,
and docs/how-to/ship-a-reviewer-lane.md; add the changeset fragment.

* fix(#3274): resolve native antigravity timeout in resolveLanePlan, not the runner

gsd-test caught two design mistakes in the prior commits:

1. SpawnPlan.argv is documented and tested as fully resolved by
   resolveLanePlan (model/effort/output/prompt already folded in) — leaving
   the antigravity '{{nativeTimeout}}' marker unresolved until the runner's
   antigravityArgv violated that contract and broke tests that read
   plan.argv directly (tests/antigravity-reviewer.test.cjs,
   tests/review-default-reviewers-workflow.test.cjs). Fix: '{{nativeTimeout}}'
   is now a fifth ARGV_PLACEHOLDER member, resolved by resolveLanePlan itself
   via the new nativeTimeoutToken() helper, exactly like the other four.
   antigravityArgv reverts to its pre-#3274 four-argument form. Also missed
   updating capabilities/antigravity/capability.json's invoke.args to match
   the descriptor, which broke the manifest/descriptor parity test.

2. tests/reviewer-config-federation.test.cjs enforces a deliberate, narrow
   invariant (#3691 narrows #2797): qwen, cursor, and coderabbit — the three
   lanes with neither a model flag nor a host — may own no config key beyond
   their own prompt-budget key. Adding review.timeouts.<slug> to all 12 lanes
   violated it. Fix: those three keep timeoutConfigKey: null and own no
   review.timeouts.* key, matching their existing modelConfigKey: null. The
   other 9 lanes are unaffected.

* chore(#3274): backfill changeset PR number (pr:0 -> 4083)

---------

Co-authored-by: sim <sim@local>

gsd-core/bin/lib/capability-validator.cjs

@@ -1824,6 +1824,9 @@ const KNOWN_REVIEWER_FIELDS = new Set([
   // missed a configured model and silently disabled the pinned-model escape hatch
   // #2073 added. A convention one shipped lane already violates is not a contract.
   'modelConfigKey',
+  // `timeoutConfigKey` added by #3274, same optional/backward-compatible shape as
+  // `modelConfigKey` above: a manifest authored before this field existed must keep validating.
+  'timeoutConfigKey',
   'handler',
 ]);
 
@@ -2371,6 +2374,17 @@ function validateReviewerBodyFields(cap) {
     );
   }
 
+  // OPTIONAL, mirroring modelConfigKey's D4 forward/backward-compat treatment (#3274): a manifest
+  // authored before this field existed must keep validating. Absent/null means "no override for
+  // this lane, use timeoutFloorMs". An empty string is neither absent nor a key, and is rejected.
+  if (r.timeoutConfigKey !== undefined && r.timeoutConfigKey !== null &&
+      (typeof r.timeoutConfigKey !== 'string' || r.timeoutConfigKey.length === 0)) {
+    errors.push(
+      ctx + ' reviewer.timeoutConfigKey must be a dotted config key or null ' +
+      '(got: ' + describeValue(r.timeoutConfigKey) + ')',
+    );
+  }
+
   // `null` is the declared "no per-lane budget"; an empty string is not.
   if (r.promptBudgetKey !== null && (typeof r.promptBudgetKey !== 'string' || r.promptBudgetKey.length === 0)) {
     errors.push(