feat(tui): add /handoff to fold context and start the next turn

[TheGreatAxios]

Summary

• Adds the /handoff slash command with optional trailing instructions. Running it arms the shared operator fold (same pipeline as compact) and delivers a next turn immediately: the instructions become the inbound content, default pivot copy when omitted.
• Unlike /compact, handoff always re-infers after the fold, so the operator can pivot goals without /clear. Pivot instructions stay sticky for the summary context and are written into the compact record.
• In-flight tool batches: handoff queues behind the batch through the serial send path; whichever boundary fires first (tool pause or pivot arrival) runs the single operator fold, because firing clears the arming. An undelivered pivot disarms.
• Targets release 0.3.31. Linear: CL-8743.

Verification

• bun test src/agent/compaction.test.ts src/tui/commands/built-in.test.ts src/tui/runner/handoff.test.ts: 83 pass, 0 fail.
• bun run check: exit 0 (lint, typecheck, dead-exports, build, projects-dir-guard incl. full suite 7844 pass, 0 fail).
• Note: worktree needed a real bun install; a symlinked node_modules made @intX resolve to the main checkout and tripped the dead-export scan floor plus a stale-incremental semver error. No repo change for this.

Fixes CL-8743

[linear-code]

CL-8743