docs(plan): TipTap-first agentic markdown collaboration

[hachej]

Summary

• Adds the TipTap-first markdown collaboration plan artifact requested by Plan: TipTap-first agentic markdown collaboration #367.
• Documents the canonical MarkdownEditCoordinator, session API, operation model, document modes, and validation guardrails.

Issues

Closes #367

Validation

• Documentation-only change; no code tests run.

Notes

• PR1 — Layout-mode seam (dropdown unchanged, no visible change) #362 is already covered by existing PR feat(core): layout-mode seam (resolveLayoutMode + CoreFront appShell slot) #364.
• PR2 — Multi-project inline left bar (Projects nav + cross-project pin) #363 remains a larger PR2 implementation dependent on the PR1 seam and no-boot session-list prerequisite from Epic: Multi-project left bar (layout modes + Projects nav) #361.

[hachej]

Kanzen triage

URL: #369
What: TipTap-first markdown collaboration plan PR for #367.
Current labels: architecture package:workspace state:active phase:review track:owner
Gate: proof
Track: owner
Proof: CI is green and the body lists validation, but no final proof comment was found.
Next action: post final proof comment, then owner plan review.
Why: docs-only does not require implementation proof, but the Kanzen proof-comment gate still needs a current auditable closeout.