memory: add task-scoped shared Markdown notes

[guicybercode]

Context

Agents need explicit, inspectable shared memory for specifications, decisions, review notes, and task state. Plain Markdown is local-first, versionable, portable, and understandable without CLI Master.

Goal

Add shared Markdown notes that can be connected to tasks and agent assignments.

Scope

• Create, rename, read, edit, and archive task notes.
• Persist metadata in SQLite and content as atomic filesystem files under an app-managed location or an explicitly chosen project path.
• Add revision checks to prevent silent last-writer-wins data loss.
• Allow agents to read and propose edits through typed daemon operations.
• Show authorship/provenance for user and agent edits.
• Support optional export into the project repository.

Acceptance criteria

• Note writes are atomic and recoverable after interruption.
• Concurrent stale edits return a conflict with the latest revision.
• Paths cannot escape the approved note root.
• Agents cannot access arbitrary files through the note API.
• Notes are never injected into a prompt without an explicit context reference.
• The UI provides accessible editing, preview, and conflict resolution.
• History records agent/session/task provenance without logging note contents.
• Tests cover Unicode, large notes, path traversal, concurrent edits, and recovery.

Out of scope

Real-time multi-user collaboration and cloud synchronization.