refactor(shared): share model-selection command choice

[juliusmarminge]

Part 1/16 of the shared-core and MCP stack. Based on t3code/codex-turn-mapping at 415ed0f73b97f1655b6282492f81d0b2bba3a9cc. Next: #10578.

The client and MCP must choose the same command when changing a thread's model. This extracts modelSelectionCommandType into the existing shared model module and makes client-runtime use it. The MCP consumer follows in its own layer.

Same provider instance uses thread.model-selection.set; a different instance uses provider.switch. The current provider instance is authoritative; changing a model name alone does not switch providers. No new provider policy or command behavior.

Validation: Existing client command and shared model suites: 25 tests passed. Client-runtime/shared typechecks passed. The client regression observes the actual command for same-provider and different-provider changes.

Layer size: 3 files, +31/-5. No MCP tools are introduced in this prerequisite.

Prepared with Codex in the OpenAI agent runtime.

Note

Extract shared modelSelectionCommandType helper for thread model selection

• Extracts the provider instance comparison logic from updateThreadMetadata into a shared modelSelectionCommandType helper in model.ts
• The helper returns thread.model-selection.set when the current and selected provider instance IDs match, and provider.switch when they differ
• Updates tests in commands.test.ts to verify command selection based on provider instance, adding a same-provider case

^{Macroscope summarized a645d4e.}

[macroscopeapp]

Approvability

Verdict: Approved at a645d4e

Macroscope's review found this PR approvable — This is a small, behavior-preserving refactor that centralizes the existing provider-instance comparison and adds coverage for both command-selection branches. The production impact is limited to calling an equivalent pure helper, with no defaults, schemas, or deployment behavior changed.

^{You can add or adjust custom eligibility rules. Learn more.}

[github-actions]

Thread transfer impact

⚠️ The latest CI run did not produce a thread transfer result for a645d4e.

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