feat(app): tag other sessions with @ and link them in messages - #57480

Summary

• Add recent-first session suggestions to @, searchable by title through the same createServerSessionEntries loader used by Home and the command palette.
• Insert title chips in the composer and serialize @session:ses_<26 alphanumeric characters>. Correct subsequent file, agent, and skill offsets; restore chips when editing or undoing a prompt.
• Share parsing and .markdown-session-link rendering between user text and assistant markdown. Known IDs show current titles; unknown IDs stay monospace. Links open the referenced session in its own tab on the current server.

History and model access

The previous implementation is in unmerged #54466 (29ad94eb2a, f22188542d, latest head 9920356280); it was not removed from beta. It used hidden metadata and appended IDs. This implementation puts the stable token directly in the message text.

A mention identifies a session; it does not attach its history. The existing SDK method is session.message.list, backed by GET /api/session/:sessionID/message. There is no built-in model tool for reading another session's history; an agent needs existing authenticated SDK/HTTP access to read it.

Verification

• bun run check passed (36 typecheck tasks; existing lint warnings).
• bun run lint:changed origin/beta passed.
• 59 focused unit tests passed across parser, linkifier, prompt request/restoration/submission, editor actions, and history.
• One composer e2e passed: select session → send stable token → click title chip → open target tab while retaining source tab.
• Captured a virtual-clock-stepped 1280×800 clip (375 frames, 60fps, 6.25s) and dark/light stills. Uploaded to the review board and verified all three media URLs return HTTP 200.
• Performance comparison is blocked: the unchanged production cold-session benchmark builds and launches but fails waitForStableTimeline at session-tab-switch-probe.ts:247. No before/after performance claim.
• First CI run: Linux/Windows builds, unit suites, and session-ui component suites passed; the new composer mention e2e passed on both platforms. Two existing session-link tests were updated for title labels and unknown-ID monospace behavior. Other app failures also appear in beta run 39472373102, including Storybook mounting, timeline shortcuts, and message colors.
• origin/beta advanced during CI. The latest direct changed-lint comparison now includes unrelated upstream changes and reports 55 findings in e2e/utils/timeline.ts and timeline-stream.spec.ts; changed lint against this branch's original base 22c3eb1241 remains clean.

Size

Production code: +327 / −82 LOC. Tests: +138 / −8 LOC. Recording scripts and artifacts are excluded from the commit. The two legacy session-link expectation updates are in #57509 (+14 / −7 test LOC); both targeted cases passed, including known-title keyboard navigation against a production build.