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>

tests/agent-install-validation.test.cjs

@@ -135,6 +135,57 @@ describe('init commands: agents_installed field (#1371)', () => {
     assert.deepStrictEqual(output.missing_agents, []);
   });
 
+  test('init reports codex when the session signal is present', () => {
+    const phaseDir = path.join(tmpDir, '.planning', 'phases', '01-setup');
+    fs.mkdirSync(phaseDir, { recursive: true });
+
+    const result = runGsdTools('init plan-phase 1 --raw', tmpDir, { CODEX_SANDBOX: 'seatbelt' });
+    assert.ok(result.success, `Command failed: ${result.error}`);
+
+    const output = JSON.parse(result.output);
+    assert.strictEqual(output.agent_runtime, 'codex');
+  });
+
+  test('init honors explicit config runtime over detection', () => {
+    const phaseDir = path.join(tmpDir, '.planning', 'phases', '01-setup');
+    fs.mkdirSync(phaseDir, { recursive: true });
+    fs.writeFileSync(
+      path.join(tmpDir, '.planning', 'config.json'),
+      JSON.stringify({ runtime: 'claude' }),
+    );
+
+    const result = runGsdTools('init plan-phase 1 --raw', tmpDir, { CODEX_SANDBOX: 'seatbelt' });
+    assert.ok(result.success, `Command failed: ${result.error}`);
+
+    const output = JSON.parse(result.output);
+    assert.strictEqual(output.agent_runtime, 'claude');
+  });
+
+  test('init is unaffected in a non-codex session', () => {
+    const phaseDir = path.join(tmpDir, '.planning', 'phases', '01-setup');
+    fs.mkdirSync(phaseDir, { recursive: true });
+
+    const result = runGsdTools('init plan-phase 1 --raw', tmpDir);
+    assert.ok(result.success, `Command failed: ${result.error}`);
+
+    const output = JSON.parse(result.output);
+    assert.strictEqual(output.agent_runtime, 'claude');
+  });
+
+  test('init honors GSD_RUNTIME over detection', () => {
+    const phaseDir = path.join(tmpDir, '.planning', 'phases', '01-setup');
+    fs.mkdirSync(phaseDir, { recursive: true });
+
+    const result = runGsdTools('init plan-phase 1 --raw', tmpDir, {
+      GSD_RUNTIME: 'opencode',
+      CODEX_SANDBOX: 'seatbelt',
+    });
+    assert.ok(result.success, `Command failed: ${result.error}`);
+
+    const output = JSON.parse(result.output);
+    assert.strictEqual(output.agent_runtime, 'opencode');
+  });
+
   test('init execute-phase includes missing_agents list when agents are missing', () => {
     const phaseDir = path.join(tmpDir, '.planning', 'phases', '01-setup');
     fs.mkdirSync(phaseDir, { recursive: true });