enhance(#3245): report the detected host runtime in init (#3307)

* test(#3245): failing-first coverage for host runtime detection in init

Locks the behavior epic #2313 Phase 5 must produce before any of it exists: init reports the detected host, explicit GSD_RUNTIME and config runtime still outrank detection, non-Codex sessions are untouched, and nothing is ever written to shared defaults (#2297).

* enhance(#3245): report the detected host runtime in init

init reported agent_runtime: claude inside a Codex session, and resolved agents_dir to the Claude agents root with agents_installed: true — a spuriously healthy triple. Runtime identity was only ever read from GSD_RUNTIME or an explicit runtime in .planning/config.json.

Adds a detection rung beneath both explicit sources, in a new pure module. Codex is identified from its own documented session environment (CODEX_SANDBOX / CODEX_SANDBOX_NETWORK_DISABLED), else an explicitly exported CODEX_HOME whose config.toml exists. The default ~/.codex is never probed: that file exists on every machine that has run Codex, so probing it would misreport other runtimes' sessions.

resolveRuntime keeps its exact contract and all 71 dependents, including formatGsdSlash command-style emission; only withProjectRoot consumes the new rung. Nothing is written on any path (#2297). Explicit config still wins (#2517).

* fix(#3245): make the parity guard real and single-source the marker

Four independent review passes found the generative-fix-divergence guard was vacuous: it asserted agreement at the one input where inferPreferredRuntime and detectHostRuntime do not differ, so it could not fail. It now pins the actual divergence point (CODEX_HOME set, config.toml absent) and records that the asymmetry is deliberate.

The config.toml marker is now single-sourced from update-context.cts and imported, rather than carried independently by two surfaces. tests/helpers.cjs now scrubs CODEX_SANDBOX and CODEX_SANDBOX_NETWORK_DISABLED: GSD reads them, so an ambient Codex session would otherwise make the non-codex control test fail non-deterministically.

Also: detection is throw-safe end to end rather than only around the fs probe; the Windows-join test is replaced with one that can actually fail (trailing-separator, catches hand-rolled concatenation); the #2297 no-write proof now wraps resolveReportedRuntime, the function that ships, across all three ladder outcomes.

* chore(#3245): backfill changeset pr number

---------

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

src/runtime-slash.cts

@@ -74,19 +74,24 @@ export function formatGsdSlash(commandName: unknown, runtime: unknown): unknown
 }
 
 /**
- * Resolve the effective runtime for a project directory.
+ * Resolve the explicit runtime for a project directory, from the two
+ * explicit sources only — no default is applied.
  *
- *   process.env.GSD_RUNTIME  >  config.runtime  >  'claude'
+ *   env.GSD_RUNTIME  >  config.runtime  >  null
  *
- * Mirrors the precedence already used by profile-output.cjs and the rest of
- * the runtime resolution chain. Returns a lowercased string so downstream
- * comparisons can be case-blind.
+ * Returns null when neither explicit source is set, so a caller can
+ * distinguish "config said claude" from "nothing was set" (needed by
+ * #3245's host-detection rung, which must only run when this returns null).
  *
  * @param projectDir - path to the project directory, or null/undefined
- * @returns the resolved runtime name
+ * @param env - environment variables to read GSD_RUNTIME from; defaults to process.env
+ * @returns the resolved runtime name, or null if neither source is set
  */
-export function resolveRuntime(projectDir: string | null | undefined): string {
-  const envRuntime = resolveRuntimeNameFromCandidates(process.env['GSD_RUNTIME']);
+export function resolveExplicitRuntime(
+  projectDir: string | null | undefined,
+  env: Record<string, string | undefined> = process.env,
+): string | null {
+  const envRuntime = resolveRuntimeNameFromCandidates(env['GSD_RUNTIME']);
   if (envRuntime) return envRuntime;
   if (projectDir) {
     try {
@@ -109,7 +114,23 @@ export function resolveRuntime(projectDir: string | null | undefined): string {
       // runtime output formatting.
     }
   }
-  return 'claude';
+  return null;
+}
+
+/**
+ * Resolve the effective runtime for a project directory.
+ *
+ *   process.env.GSD_RUNTIME  >  config.runtime  >  'claude'
+ *
+ * Mirrors the precedence already used by profile-output.cjs and the rest of
+ * the runtime resolution chain. Returns a lowercased string so downstream
+ * comparisons can be case-blind.
+ *
+ * @param projectDir - path to the project directory, or null/undefined
+ * @returns the resolved runtime name
+ */
+export function resolveRuntime(projectDir: string | null | undefined): string {
+  return resolveExplicitRuntime(projectDir) ?? 'claude';
 }
 
 /**