fix(#2832): gsd-sdk auto detects Codex runtime correctly (#2844)

* fix(#2832): gsd-sdk auto detects Codex runtime correctly

Two-part fix for #2832 (gsd-sdk auto silently routing non-Claude runtime
projects through the Claude Agent SDK):

1. Runtime gate at the `auto` entry point. New `runtime-gate.ts` exports
   `assertRuntimeSupportsAutoMode(config)` which throws an actionable error
   when `GSD_RUNTIME` / `config.runtime` resolves to a non-Claude runtime
   (codex, gemini, opencode, etc.). The autonomous orchestrator only knows
   how to drive `@anthropic-ai/claude-agent-sdk` today; failing fast with a
   clear pointer at the in-session slash commands beats the previous instant
   `[FAILED] $0.00 0.1s` flake. Wired into `cli.ts` before the GSD/InitRunner
   construction.

2. Runtime-aware `resolveModel()` in `session-runner.ts`. The profile -> id
   map (`balanced -> claude-sonnet-4-6`, etc.) was applied unconditionally,
   so even with `runtime: codex` and `resolve_model_ids: omit` the SDK
   forced a Claude id into `query()`. Now the profile map only fires when
   the runtime is Claude and the explicit `resolve_model_ids: "omit"` knob
   short-circuits to undefined, mirroring `query/config-query.ts`.

Tests (vitest, sdk/src):
- runtime-gate.test.ts (8 cases): claude / unset / unknown pass; codex,
  gemini, opencode throw; GSD_RUNTIME wins over config.runtime; error
  message references #2832 and the slash-command workaround.
- session-runner.test.ts (4 new cases under "resolveModel runtime
  awareness (#2832)"): codex runtime + balanced profile -> no model
  injected; resolve_model_ids: omit -> no model; claude runtime still
  resolves to claude-sonnet-4-6 (no regression); explicit options.model
  wins on any runtime.

* fix(#2832): address CR — env-precedence in resolveModel + accurate source attribution

Two CodeRabbit findings on PR #2844:

1. session-runner.ts:resolveModel() (Major) — read runtime via detectRuntime()
   so GSD_RUNTIME env precedence is honored. Without this, a Codex run with
   a Claude-shaped config still fell into the Claude-only profile-id branch.

2. runtime-gate.ts:assertRuntimeSupportsAutoMode() (Minor) — when GSD_RUNTIME
   holds an unsupported value, detectRuntime() falls through to config but
   the source label still reported the discarded env value. Fix: validate
   env against SUPPORTED_RUNTIMES before attributing the source.

Tests added for both: env-precedence in session-runner, source attribution
in runtime-gate. 17/17 pass.

sdk/src/session-runner.test.ts

@@ -95,4 +95,70 @@ describe('runPhaseStepSession', () => {
     expect(result.success).toBe(true);
     expect(result.sessionId).toBe('test-session');
   });
+
+  // ─── #2832: runtime-aware model resolution ─────────────────────────────────
+  // Issue: with `runtime: codex` and `model_profile: balanced`, resolveModel
+  // mapped balanced -> 'claude-sonnet-4-6' and forced the Codex run through
+  // a Claude model. For non-Claude runtimes the SDK must NOT inject a Claude
+  // model id; it should leave model unset (or honor explicit overrides) and
+  // let the runtime use its configured default.
+  describe('resolveModel runtime awareness (#2832)', () => {
+    it('does NOT pass a Claude profile model id when runtime is codex', async () => {
+      await runPhaseStepSession(
+        'prompt',
+        PhaseStepType.Execute,
+        makeConfig({ runtime: 'codex', model_profile: 'balanced' } as Partial<GSDConfig>),
+      );
+      const opts = mockQueryCalls[0].options as { model?: string };
+      expect(opts.model).toBeUndefined();
+    });
+
+    it('does NOT pass a Claude profile model id when resolve_model_ids is "omit"', async () => {
+      await runPhaseStepSession(
+        'prompt',
+        PhaseStepType.Execute,
+        makeConfig({ resolve_model_ids: 'omit', model_profile: 'balanced' } as Partial<GSDConfig>),
+      );
+      const opts = mockQueryCalls[0].options as { model?: string };
+      expect(opts.model).toBeUndefined();
+    });
+
+    it('still maps profile -> Claude id when runtime is claude (no regression)', async () => {
+      await runPhaseStepSession(
+        'prompt',
+        PhaseStepType.Execute,
+        makeConfig({ runtime: 'claude', model_profile: 'balanced' } as Partial<GSDConfig>),
+      );
+      const opts = mockQueryCalls[0].options as { model?: string };
+      expect(opts.model).toBe('claude-sonnet-4-6');
+    });
+
+    it('respects GSD_RUNTIME env precedence over config (no Claude id when env=codex)', async () => {
+      const prev = process.env.GSD_RUNTIME;
+      process.env.GSD_RUNTIME = 'codex';
+      try {
+        await runPhaseStepSession(
+          'prompt',
+          PhaseStepType.Execute,
+          makeConfig({ model_profile: 'balanced' } as Partial<GSDConfig>),
+        );
+        const opts = mockQueryCalls[0].options as { model?: string };
+        expect(opts.model).toBeUndefined();
+      } finally {
+        if (prev === undefined) delete process.env.GSD_RUNTIME;
+        else process.env.GSD_RUNTIME = prev;
+      }
+    });
+
+    it('explicit options.model wins for any runtime', async () => {
+      await runPhaseStepSession(
+        'prompt',
+        PhaseStepType.Execute,
+        makeConfig({ runtime: 'codex' } as Partial<GSDConfig>),
+        { model: 'gpt-5.3-codex' },
+      );
+      const opts = mockQueryCalls[0].options as { model?: string };
+      expect(opts.model).toBe('gpt-5.3-codex');
+    });
+  });
 });