fix: store UI-selected model for first message model inheritance

[LTS2]

Summary

• Fix Sisyphus to use the model selected via UI before the first message is sent
• Add session model storage to capture model info in chat.message hook
• Use stored session model as fallback in delegate_task when prevMessage is unavailable

Changes

• state.ts: Add SessionModel interface and storage functions (setSessionModel, getSessionModel, clearSessionModel, updateSessionModel)
• index.ts: Store UI-selected model in chat.message hook, cleanup in session.deleted event
• tools.ts: Use stored session model as fallback for first message scenario

Test plan

• Added 7 unit tests (67 pass, 1 skip, 0 fail)
• Manual test: Select model in new session, trigger Sisyphus on first message, verify selected model is used

Fixes #1024

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Summary by cubic

Fixes model inheritance at session start so the UI-selected model is used on the first message, preventing Sisyphus from defaulting to the wrong model.

• Bug Fixes
  • Persist the session’s selected model (first-write wins) with set/get/update/clear helpers.
  • Save the UI-selected model in the chat.message hook and clear it on session.deleted.
  • Use the stored session model as fallback in delegate_task when prevMessage is not available.
  • Added unit tests covering model storage and the first-message fallback scenario.

^{Written for commit 4f84007. Summary will update on new commits.}

[cubic-dev-ai]

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Confidence score: 5/5

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