docs(mcp): document the MCP surface, authoring controls, and skill (v0.8.0)

Document the per-graph MCP surface (POST /graphs/{id}/mcp, shipped in the
preceding commits and landing under v0.8.0) and the `.gq` authoring controls
that shape stored-query tools.

- New docs/user/operations/mcp.md: the client-facing guide — transport, tool
  catalog (built-ins + stored queries), projection modes, structured output,
  authorization (call-authoritative + list-relaxation), Host/Origin policy, the
  protocol-version contract.
- docs/user/operations/server.md: the /mcp endpoint + an "MCP surface" section;
  docs/user/index.md: a "Connect an MCP agent" pointer.
- docs/user/queries/index.md: an Annotations section — query @description /
  @instruction / @mcp(expose, tool_name) and per-parameter @description.
- AGENTS.md: topic-table row + MCP note on the HTTP-server capability row.
- docs/dev/testing.md: the omnigraph-mcp crate + server tests/mcp.rs.
- docs/dev/rfc-005 §D5: retire the "cluster = everything exposed" bridge —
  cluster mode honors source `@mcp(expose: …)`; presentation vs authorization
  split made explicit.
- skills/omnigraph: server-policy.md MCP section; stored-queries.md corrected
  (per-query controls now ship via @mcp, not "planned"); SKILL.md MCP triggers,
  Deep Dives row, version → 0.8.0.
- docs/releases/v0.8.0.md: the MCP surface + authoring-controls release notes.

Crate version manifests are deliberately NOT bumped — that is the v0.8.0
release-cut step; this lands on the feature branch.

docs/user/operations/server.md

@@ -46,6 +46,7 @@ graph id from the cluster's applied revision:
 | POST | `/graphs/{id}/change` | bearer + `change` | **deprecated** alias of `/mutate` (carries `Deprecation: true` + `Link: <mutate>; rel="successor-version"`) |
 | GET | `/graphs/{id}/queries` | bearer + `read` | list the `mcp.expose` stored queries as a typed tool catalog |
 | POST | `/graphs/{id}/queries/{name}` | bearer + `invoke_query` (+ `change` for a stored mutation) | invoke a named query from the `queries:` registry; deny == 404 |
+| POST | `/graphs/{id}/mcp` | bearer + same per-tool Cedar gate | MCP (Model Context Protocol) surface — built-ins + stored queries as tools, schema/branches as resources (see [mcp.md](mcp.md)) |
 | GET | `/graphs/{id}/schema` | bearer + `read` | get current `.pg` source |
 | POST | `/graphs/{id}/schema/apply` | bearer + `schema_apply` (target=`main`) | disabled for cluster-backed serving; returns 409 and points operators at `omnigraph cluster apply` + restart |
 | POST | `/graphs/{id}/load` | bearer + `branch_create` (only when `from` is set and the branch is created) + `change` | bulk load (canonical); branch creation is opt-in via `from` — without it a missing `branch` is a 404, never an implicit fork (32 MB body limit) |
@@ -65,7 +66,7 @@ Server-level management endpoints:
 
 ### Stored-query catalog (`GET /queries`)
 
-List the graph's **`mcp.expose`** stored queries as a typed tool catalog — enough for a client (e.g. an MCP server) to register each as a tool without fetching `.gq` source. Each entry: `{ name, tool_name, description, instruction, mutation, params }`, where each param is `{ name, kind, item_kind?, vector_dim?, nullable }`. `kind` is one of `string | bool | int | bigint | float | date | datetime | blob | vector | list` (decomposed so a consumer maps it with a closed `switch`, never re-parsing GQ type spelling). `bigint` (I64/U64), `date`, `datetime`, and `blob` are carried as JSON **strings** — a 64-bit integer loses precision as a JSON number, dates are ISO strings, and a blob is a URI string.
+List the graph's **`mcp.expose`** stored queries as a typed tool catalog — enough for a client to register each as a tool without fetching `.gq` source. (The server also projects these queries as live MCP tools at `POST /graphs/{id}/mcp` — see [mcp.md](mcp.md); this catalog endpoint is the REST view of the same registry.) Each entry: `{ name, tool_name, description, instruction, mutation, params }`, where each param is `{ name, kind, item_kind?, vector_dim?, nullable }`. `kind` is one of `string | bool | int | bigint | float | date | datetime | blob | vector | list` (decomposed so a consumer maps it with a closed `switch`, never re-parsing GQ type spelling). `bigint` (I64/U64), `date`, `datetime`, and `blob` are carried as JSON **strings** — a 64-bit integer loses precision as a JSON number, dates are ISO strings, and a blob is a URI string.
 
 - **Read-gated** (works in default-deny mode). The catalog is **graph-wide** (branch-independent; `read` is authorized against `main`).
 - **`mcp.expose` defaults to `true`** — declaring a query in `queries:` lists it; set `mcp: { expose: false }` to keep it HTTP/service-callable but hidden from the catalog.
@@ -80,6 +81,22 @@ Invoke a curated, server-side stored query by **name** — the source comes from
 - **Requires an explicit policy grant when auth is on.** In default-deny mode (bearer tokens but no `policy.file`), only `read` is permitted, so *every* `/queries/{name}` call returns `404` until an `invoke_query` rule is configured.
 - A stored mutation cannot target a `snapshot` (`400`); a parameter type error is a structured `400` naming the parameter.
 
+## MCP surface (`POST /graphs/{id}/mcp`)
+
+Each served graph is also an MCP (Model Context Protocol) server at
+`POST /graphs/{id}/mcp` — a stateless Streamable-HTTP transport that projects the
+built-in operations and the graph's stored-query registry as MCP **tools**, and
+the schema / branch list as MCP **resources**. It adds no new capability: every
+tool delegates to the same engine/handler path the REST routes use and is gated
+by the same Cedar policy (resolved from the same bearer token). `tools/list` is a
+*relaxation* of the per-call gate — a tool callable on some branch is never
+hidden, while the per-call gate stays authoritative. Served automatically by the
+cluster server; no separate flag.
+
+Full client guide — connecting, the tool catalog, projection modes, structured
+output, Host/Origin policy, and the protocol-version contract — is in
+[mcp.md](mcp.md).
+
 ## Adding and removing graphs
 
 Runtime add/remove via API is **not** exposed — neither `POST /graphs`