Add conversation message branching

[JeffreyDeng-spec]

Summary

• Add persisted conversation branch metadata for message branches, including fork source task/session/provider and branch lookup helpers.
• Add branch creation/origin APIs and first-branch-message runtime context so historical message branches fork from the latest completed parent runtime session while preserving the selected root message.
• Update web chat branching UI, branch quote/default context, and branch sheet return state; update CLI Claude/Codex session fork handling.

Tests run

• git diff --cached --check
• pnpm --filter @alook/shared test -- test/queries/conversation-branch.test.ts test/queries/conversation.test.ts test/queries/message.test.ts test/queries/task.test.ts
• pnpm --filter @alook/web test -- 'src/app/api/conversations/[id]/branches/route.test.ts' 'src/app/api/conversations/[id]/branch-origin/route.test.ts' 'src/app/api/conversations/[id]/messages/route.test.ts' src/components/agent-chat/agent-chat-view.test.ts src/components/agent-chat/message-list.test.ts src/contexts/agent-chat-sheet-state.test.ts
• pnpm --filter @alook/cli test -- src/daemon/agent/__tests__/claude.test.ts src/daemon/agent/__tests__/codex.test.ts src/daemon/session-runner.test.ts
• pnpm typecheck
• pnpm lint
• CI=1 pnpm test
• Pre-commit hook reran pnpm typecheck && pnpm lint && pnpm test successfully.

Known residual risk

• No known blockers. Residual risk is limited to no live production/staging browser smoke in this handoff; branch behavior is covered by targeted and broad automated tests.

Main branch note

• No GitHub main content was directly changed. This work is isolated to feat/release-branch-feature-rebase and this PR.

Supersedes

• Supersedes Add conversation message branching #306, which used the wrong tony/ head branch prefix and was closed after the branch was renamed to the required feat/ prefix.