feat(composer): add past chats reference flow

[SuMuxi66]

Closes #3185

Summary

Add @ → past:chats flow to the Composer, allowing users to select a historical chat session and inject its context into the current conversation.

What's included

• @ menu: New past:chats entry in the @ context menu
• Session list: Browse recent chat sessions with title and turn count
• Search: Filter sessions by keyword
• Hover preview: Preview session content on hover before selecting
• Context injection: Selected session's conversation is appended to the prompt as context
• Architecture doc: docs/SESSION_REFERENCE_ARCHITECTURE.md documenting the data flow

Files changed

File	Change
Composer.tsx	@ menu entry, session list, search, hover preview, reference card in Composer
FileMenu.tsx	Wire past:chats menu item
types.ts	SessionReference interface
styles.css	Composer reference card styles
SESSION_REFERENCE_ARCHITECTURE.md	Architecture documentation

Test checklist

• @ menu shows past:chats option
• Clicking past:chats opens session list
• Search filters sessions by keyword
• Hover preview shows session content
• Selecting a session adds reference card above Composer input
• Reference card shows title, turn count, file name
• Hover on reference card shows full path
• Sending with reference: AI can reference the selected session content
• Sending without reference: normal behavior unchanged
• Deleting reference card before send: no reference injected
• @ file references still work
• / command menu still works
• No regression in existing Composer behavior

[SuMuxi66]

Scope

This PR only adds the past:chats reference flow to the desktop Composer UI (desktop/frontend). It does not change the CLI/TUI input flow.

[SivanCola]

Thanks, this is directionally useful, but I don't think it is ready to merge yet.

Please address these before merging:

• The new past:chats entry is rendered inside the @ menu, but the menu state machine still only counts and picks file entries. menuMode only opens when atMatches.length > 0, count is still atMatches.length, and pickActive() still routes Enter/Tab to pickEntry(atMatches[active]). That means keyboard selection cannot open past:chats, keyboard selection inside the past-chat list cannot choose a session, and the entry can disappear entirely when there are no file matches. Please make past:chats a real selectable item in the @ menu state model, including keyboard navigation and the empty-file-list case.

• submit() now awaits buildSessionContext(sessionRefs) before calling onSend, but there is no local submitting guard while those PreviewSession calls are pending. During that window, the send button remains enabled and Enter can trigger another submit, so users can duplicate the same message if session preview reads are slow. Please add a pending-submit state or otherwise disable duplicate sends until the async context build finishes.

• git diff --check currently fails due to trailing whitespace in docs/SESSION_REFERENCE_ARCHITECTURE.md at lines 71, 144, 164-166, 173, 200, 209, and 219.

Notes from review:

• I did not see new dependency, network, command execution, or provider/tool-schema cache-stability risk.
• The PR is behind latest main-v2, but a merge-tree check against the latest base did not show a text conflict. Please still rebase or update before final verification.
• Frontend typecheck could not be completed in a temporary worktree because this checkout lacks generated Wails wailsjs types; that is not counted as a PR-specific failure here.

Once these are addressed and CI stays green, I will consider merging it into main-v2.