feat(mcpsrv): spec_propose write tool (Phase 3)

The agent-facing half of the write plane over MCP. spec_propose uploads
whole documents and returns {proposal, url, merged}, calling the same
service.Propose the REST PUT will — one implementation of If-Match,
provenance and auto-merge behind both surfaces, never two that drift.

- Writer is an optional Backend field: nil keeps the server read-only
  (the three read tools, unchanged), so a read-only deploy or a test
  needs no mutable backend. Set, it registers spec_propose with neither
  the read-only nor the idempotent hint — proposing twice opens two
  proposals.
- The acting agent is resolved from the bearer token on the tool call by
  the resolver middleware now installed on /mcp; the read tools never
  needed it. The ACL stays in service/: service.Propose refuses a
  non-agent, so an anonymous or owner caller is rejected there.