feat(composer): add past chats reference flow

[SuMuxi66]

Part of #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.

[SuMuxi66]

Updated.

Changes addressed:

• Made past:chats a first-class selectable @ menu item, including keyboard navigation and root-level empty-file-list behavior.
• Added pending-submit guard with ref + state so async buildSessionContext(sessionRefs) cannot duplicate sends.
• Cleaned trailing whitespace in docs/SESSION_REFERENCE_ARCHITECTURE.md.
• Rebased onto latest main-v2.
• Verified git diff --check passes and manually tested @, past:chats, file refs, / menu, and duplicate-submit prevention.

[SuMuxi66]

Updated again.

Rebased onto the latest main-v2 and resolved the new styles.css conflict.

Verified:

• git diff --check origin/main-v2..HEAD passes.
• git merge-tree --write-tree origin/main-v2 HEAD returns a clean tree with no conflicts.
• The PR still only touches the intended files:
  • Composer.tsx
  • FileMenu.tsx
  • types.ts
  • styles.css
  • docs/SESSION_REFERENCE_ARCHITECTURE.md

Manual checks still pass for root-level @, past:chats, file refs, / menu, session selection focus return, and duplicate-submit prevention.

[SivanCola]

Thanks for the updates. The previous blocking items are mostly addressed, but I found one remaining regression that should be fixed before merge:

• The root-level @ menu now always inserts past:chats as the first item whenever atDir === "". Because active resets to 0 as the query changes, typing something like @README or @src and pressing Enter/Tab opens the past-chats picker instead of selecting the first matching file entry. This regresses the existing @file keyboard flow and conflicts with the PR checklist item that file references still work. Please only show/select past:chats when the empty root @ menu is open or when the current fragment actually matches past:chats, or otherwise preserve the first file match as the default active item for non-empty file queries.

Also, please avoid closing #3185 from this PR as-is. The issue asks for a broader #past_chats flow and is tagged for desktop/TUI/agent, while this PR explicitly scopes itself to the desktop Composer. Part of #3185 or a follow-up issue for the remaining TUI/trigger behavior would be more accurate.

Once the @file keyboard regression is fixed and verification stays green, I will consider merging this into main-v2.

[SivanCola]

Approved. The latest update addresses the remaining blocker from my previous review: past:chats is no longer unconditionally inserted ahead of file matches for non-matching @ fragments, so normal @README / @src style keyboard selection can keep resolving to file entries while @ and matching @past... fragments can still reach the past-chats picker.

Why this fits the architecture:

• The feature stays scoped to the desktop Composer and uses the existing ListSessions / PreviewSession bridge methods instead of adding new persistence, network, command execution, or permission surfaces.
• Referenced chat content is added only to the dynamic submit payload after an explicit user selection; it does not change provider-visible stable prefixes, tool schemas, provider serialization, or cache-sensitive prompt setup.
• The PR body now correctly says Part of #3185, which matches the desktop-only scope and leaves the broader TUI / trigger behavior open for follow-up.

Related work: #3338 is still the stacked follow-up for rendering selected references inside user message bubbles, so it should be rebased/reviewed after this core Composer flow lands.

Verified:

• git diff --check origin/main-v2...origin/pr/3336 passed
• git merge-tree --write-tree origin/main-v2 origin/pr/3336 produced a clean tree
• pnpm --dir desktop/frontend check:css passed
• pnpm --dir desktop/frontend test:all passed (85 tests)

Caveat: pnpm --dir desktop/frontend typecheck still cannot complete in a temporary checkout because the Wails-generated wailsjs bindings are absent there; this is the same environment limitation as before, not a PR-specific failure.