MCP server v1: outside agents read and propose reviewable edits

[tosinamuda]

User story
As a Claude Desktop or Cursor user, I want my agent to read and safely propose edits to my Compose workspace, so that Compose's review and version history protect me even when the agent lives outside Compose.

Blocked by the MCP spike (#127) — scope inherits its decision doc.

Acceptance criteria

• Registering Compose in Claude Desktop takes ≤ 2 steps (per spike's setup UX)
• The agent can read and search workspace notes
• An agent edit NEVER lands silently: it arrives as a reviewable change (accept/reject) or snapshot-backed write, and appears in version history
• Workspace-scoped: no path outside the registered workspaces is readable or writable
• Documented in README + positioning doc (this is a differentiator)

Sub-tasks

• (from spike decision doc)
• Live verify with real Claude Desktop end-to-end