Navigate pinned and saved chat items

[rogerchappel]

Summary

• cap channel pins at the latest three items and render them as top pinned shortcuts
• deep-link pinned messages and saved Later items back into the correct chat/session/message
• persist chat item agent/session metadata when bookmarking so Later can route back to Neo/subagent contexts

Verification

• pnpm typecheck
• pnpm lint (0 errors, existing warnings remain)
• pnpm build
• git diff --check origin/main..HEAD

Risk

Low to medium: touches chat navigation state and saved/pinned item UI only; no schema or migration changes.