feat(flow-chat): reference idle sessions from chat input

[wsp1911]

Summary

• Extend the chat input @ picker to search files, folders, and idle sessions across currently open local and SSH workspaces.
• Send selected session locators as structured message metadata and preserve that metadata through the frontend queue.
• Materialize bounded, newest-first session transcripts into current-session artifacts only when a turn is dispatched.
• Add short collision-safe transcript artifact names, line-range metadata, and a safe-use reminder for agents.
• Reject archived, hidden, busy, queued, invalid, or self-referencing sessions.

Fixes: N/A

Type and Areas

Type:

Feature / UI/UX

Areas:

Rust core, desktop/Tauri, web UI, session persistence

Motivation / Impact

Users can provide another idle session as context without exposing that session's storage directory to agent tools. The agent receives a bounded local artifact and exact index/latest-turn line ranges for targeted Read or Grep calls.

Verification

• pnpm run fmt:rs
• pnpm run type-check:web
• cargo check -p bitfun-desktop
• cargo test -p bitfun-core session_reference_artifact_stems_extend_only_for_collisions -- --nocapture
• cargo test -p bitfun-core materialized_session_reference_keeps_newest_complete_turn_and_overwrites_artifact -- --nocapture
• git diff --check

Reviewer Notes

• Session search is limited to currently open workspaces, including SSH workspaces.
• Referenced transcripts are stored only under the consuming session's artifacts/session-references directory.
• Normal filenames use the first 8 session-ID characters; same-turn prefix collisions extend by 4 characters.
• Reminder metadata includes the full session ID, transcript URI, index range, latest-turn range, and total line count.
• Transcript contents are explicitly treated as untrusted historical data.

Checklist

• This PR is focused and does not include secrets, temporary prompts, generated scratch files, or unrelated artifacts.
• Relevant verification is recorded above.
• User-facing strings, docs, and locales are updated where applicable.