refactor(authn): one Principal.CanRead() for the read-plane ACL (spec-ejq.1)

graph's gate, web's mayRead and mcpsrv's Gate each hand-spelled
'IsOwner() || IsAgent()' — three copies of the read ACL, which graph's own
comment warned is how a corpus leaks when they drift. Define it once as
authn.Principal.CanRead and route all three through it.

coreauth.Derive keeps its own owner||agent test on purpose: it answers a
different question (is this an owner-backed identity to bridge to
AUTH_INTERNAL), and coupling it to the read ACL would misroute a future
read-only viewer kind to the owner's UserID.

Closes spec-ejq.1

fix(mcpsrv): gate read tools to owner+agents (spec-jjo)

The MCP read tools (spec_search/spec_read/spec_list) enforced no ACL:
anyone clearing the Host allowlist could read approved content. graph's
/query and the web UI gate reads to owner+agents; /mcp did not. Add the
same fail-closed gate to the whole MCP surface — initialize, tools/list
and tools/call — mounted inside the resolver middleware so it sees the
resolved principal. spec_propose was already fail-closed in service.Propose;
this makes the read tools match.

Pre-existing since Phase 2, cheap now that /mcp resolves a principal.

Closes spec-jjo

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.

feat(mcpsrv): MCP read tools, with Host validation replacing the SDK guard

Adds the Phase 2 read tools (spec_search, spec_read, spec_list) over the
service layer.

Two security fixes came out of building them.

The read plane could serve proposal content. gitx resolves ref names, and
service.resolveRev passed any string through, so rev=proposals/42 made the
READ plane hand back unreviewed text — which would then flow into agent
context as though approved, the single failure this service exists to
prevent. ValidateReadRev now admits only the approved-head sentinel or a
full 40-character object name, at the layer all three surfaces share.
Abbreviations are refused too: one that is unique today can become
ambiguous later, so a pinned revision would silently stop meaning one
thing.

The MCP SDK's DNS-rebinding guard rejects a loopback listener whose Host
is not loopback, which is exactly nginx forwarding to 127.0.0.1 — it would
403 only in production, passing every local test. The SDK offers no
allowlist, so the guard is disabled and replaced by a stricter check: Host
must equal the configured origin, or a loopback name for development. A
rebinding attack carries the attacker's name in Host and fails it. An
unusable origin logs loudly rather than quietly unguarding the endpoint.