mr-668: remove POST /graphs and CLI graphs create (defer runtime graph mgmt)

The POST /graphs runtime-create endpoint shipped in PR 7/10 has three
unresolved high-severity bugs:

  - flock-on-renamed-inode race: the YAML flock is taken on
    omnigraph.yaml itself, then a temp file is renamed over it.
    Cross-process writers end up locking different inodes — both
    believing they hold exclusive access.
  - duplicate-check outside the file lock: precheck runs against
    the in-memory registry only; the locked closure does
    config.graphs.insert(...) unconditionally. Concurrent same-id
    POSTs can persist the loser in YAML while the in-memory registry
    keeps the winner — they disagree after restart.
  - best_effort_cleanup_init_artifacts deletes _schema.pg /
    _schema.ir.json / __schema_state.json on any init failure. An
    accidental re-init against an existing graph's URI destroys its
    schema; subsequent open() fails at read_text(_schema.pg).

The correct fix is a Lance-style cluster catalog (reserve → init →
publish with recovery sidecars), parallel to the engine's existing
__manifest discipline. That work is out of scope for v0.7.0.

For now, disable runtime add/remove from the network and CLI surface.
Operators add graphs by editing omnigraph.yaml and restarting. The
GET /graphs read-only enumeration stays.

Removed:
- POST /graphs handler + router fragment + utoipa registration
- 13 post_graphs_* server tests + 3 composite POST tests +
  multi_mode_app_with_real_config / post_graph helpers
- CLI omnigraph graphs create subcommand + its handler + cli.rs tests
- system_remote.rs combined list+create test trimmed to list-only
- YAML rewrite infra: rewrite_atomic[_with_modify], RewriteAtomicError,
  staging_path, hash_config_file, AppState::config_hash field +
  threading through new_multi and open_multi_graph_state
- fs2 dependency (verified absent from cargo tree)
- sha2/fs2 imports in config.rs (only the rewrite path used them)
- Cedar PolicyAction::GraphCreate variant + "graph_create" match arms
  + action def in Cedar schema + graph_create_action_authorizes_against_server_resource test
- GraphCreateRequest / GraphCreateResponse / GraphSchemaSpec /
  GraphPolicySpec API types (only the POST handler / CLI imported them)

Kept:
- GET /graphs (read-only enumeration) and graph_list Cedar action
- omnigraph graphs list CLI subcommand
- All multi-graph startup, mode inference, cluster routes,
  per-graph + server-level Cedar policies
- server_settings_drive_multi_graph_startup_end_to_end (the test
  that covers operator-authored YAML + restart — the path that
  survives)
- best_effort_cleanup_init_artifacts and the three init failpoints
  (still reachable from CLI `omnigraph init`; preflight fix deferred
  as a follow-up)
- GraphRegistry::insert and its concurrency tests — production
  callers gone, but the method is the natural seam for the future
  cluster-catalog work

Also fixed (transcript issue 4):
- ALWAYS_FLAT_PATHS now includes /graphs so multi-mode OpenAPI
  advertises the management route correctly (was previously rewritten
  to /graphs/{graph_id}/graphs)
- multi_mode_openapi_keeps_healthz_flat → renamed to
  multi_mode_openapi_keeps_management_paths_flat, asserts both
  /healthz and /graphs stay flat
- multi_mode_openapi_prefixes_operation_ids_with_cluster skips
  /graphs in addition to /healthz

Doc fixes:
- docs/user/cli.md: graphs list example was --target http://...,
  but --target is a config-graph-name lookup; corrected to --uri.
  Removed the graphs create example.
- docs/user/server.md: dropped POST /graphs row, "omnigraph.yaml
  ownership", and "POST /graphs body shape" sections. Added a
  paragraph stating runtime add/remove is not exposed in v0.7.0.
- docs/user/policy.md: dropped graph_create action; reworded the
  "Configuration" line to clarify that server-scoped rules (graph_list)
  take neither branch_scope nor target_branch_scope.
- docs/releases/v0.7.0.md: rewrote release narrative — multi-graph
  mode ships; runtime add/remove deferred.
- AGENTS.md: HTTP server bullet and capability matrix row updated to
  reflect read-only GET /graphs and the operator-edit workflow.
- openapi.json regenerated; /graphs has only .get, no .post.

Diff: 17 files, +123 −1525 LOC.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/user/server.md

@@ -47,34 +47,18 @@ Server-level management endpoints (v0.7.0+):
 | Method | Path | Auth | Action | Handler |
 |---|---|---|---|---|
 | GET | `/graphs` | bearer + `graph_list` on `Server::"root"` | list registered graphs | `server_graphs_list` (405 in single mode) |
-| POST | `/graphs` | bearer + `graph_create` on `Server::"root"` | create new graph at runtime | `server_graphs_create` (405 in single mode, 32 MB body limit) |
 
-`DELETE /graphs/{id}` is **not** in v0.7.0. Operators remove graphs by stopping the server, editing `omnigraph.yaml`, then restarting.
+## Adding and removing graphs (multi mode)
 
-## `omnigraph.yaml` ownership (multi mode)
+Runtime add/remove via API is **not** exposed in v0.7.0 — neither
+`POST /graphs` nor `DELETE /graphs/{id}` is implemented. Operators add
+or remove graphs by stopping the server, editing the `graphs:` map in
+`omnigraph.yaml`, then restarting. The server treats `omnigraph.yaml`
+as operator-owned configuration and never writes it.
 
-The server owns `omnigraph.yaml` while running. `POST /graphs` rewrites the file atomically under an exclusive `fcntl::flock` with SHA-256 drift detection:
-
-- The server hashes the file at startup. `POST /graphs` re-hashes under the flock before rewriting. If the hash doesn't match (operator hand-edited), the rewrite refuses with 503.
-- Comments and blank-line structure are **not** preserved across server-side rewrites — the file is regenerated via `serde_yaml::to_string`.
-- Operators must not edit the file while the server is running. To make offline changes: stop the server, edit, restart.
-
-In **single mode** the server never writes `omnigraph.yaml`.
-
-## `POST /graphs` body shape
-
-```json
-{
-  "graph_id": "alpha",
-  "uri": "s3://tenant-bucket/alpha",
-  "schema": { "source": "<inline .pg source>" },
-  "policy": { "file": "./policies/alpha.yaml" }
-}
-```
-
-- `schema` and `policy` are nested — leaves room for future fields without breaking the shape.
-- `policy` is optional; without it, no per-graph Cedar enforcement.
-- Status codes: 201 Created · 400 invalid body · 401 missing bearer · 403 Cedar denied · 405 single mode · 409 duplicate `graph_id` or `uri` · 413 body >32 MiB · 500 init or rewrite failure · 503 YAML drift.
+A future release may introduce a managed registry (Lance-backed,
+catalog-style: reserve → init → publish with recovery sidecars) and
+re-expose runtime mutation on top of it.
 
 ## Streaming