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/cli.ts

@@ -17,6 +17,8 @@ import { CLITransport } from './cli-transport.js';
 import { WSTransport } from './ws-transport.js';
 import { InitRunner } from './init-runner.js';
 import { validateWorkstreamName } from './workstream-utils.js';
+import { loadConfig } from './config.js';
+import { assertRuntimeSupportsAutoMode } from './runtime-gate.js';
 
 // ─── Parsed CLI args ─────────────────────────────────────────────────────────
 
@@ -556,6 +558,18 @@ export async function main(argv: string[] = process.argv.slice(2)): Promise<void
 
   // ─── Auto command ─────────────────────────────────────────────────────────
   if (args.command === 'auto') {
+    // #2832: refuse to silently route non-Claude runtime projects through the
+    // Claude Agent SDK. Load project config (best effort — falls back to
+    // defaults when missing) and gate before constructing GSD/InitRunner.
+    try {
+      const cfg = await loadConfig(args.projectDir, args.ws);
+      assertRuntimeSupportsAutoMode(cfg);
+    } catch (err) {
+      console.error(`Fatal error: ${(err as Error).message}`);
+      process.exitCode = 1;
+      return;
+    }
+
     const gsd = new GSD({
       projectDir: args.projectDir,
       model: args.model,