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/cli.md

@@ -46,26 +46,17 @@ and configure the matching `bearer_token_env` in `omnigraph.yaml`.
 
 ## Multi-graph servers (v0.7.0+)
 
-Against a multi-graph server (started with `--config omnigraph.yaml` referencing a non-empty `graphs:` map), use `omnigraph graphs` to enumerate and create graphs:
+Against a multi-graph server (started with `--config omnigraph.yaml` referencing a non-empty `graphs:` map), use `omnigraph graphs list` to enumerate the registered graphs:
 
 ```bash
-# List
-omnigraph graphs list --target http://server.example.com --json
-
-# Create
-omnigraph graphs create \
-  --target http://server.example.com \
-  --graph-id beta \
-  --graph-uri /data/beta.omni \
-  --schema schema.pg \
-  --policy-file ./policies/beta.yaml   # optional
+omnigraph graphs list --uri http://server.example.com --json
 ```
 
-The CLI reads `--schema` from the local disk and inlines the contents as `schema.source` in the request body. Both subcommands reject local URI targets — they're for remote multi-graph servers only.
+`list` rejects local URI targets — it's for remote multi-graph servers only.
 
-`omnigraph graphs delete` is **not** in v0.7.0. To remove a graph, stop the server, edit `omnigraph.yaml`, restart.
+Runtime add/remove is **not** in v0.7.0. To add a graph, stop the server, add a `graphs.<id>` entry to `omnigraph.yaml`, then restart. To remove, stop the server, delete the entry, restart.
 
-Per-graph URLs: once a graph exists, hit its cluster route from any subcommand by pointing `--uri` at it:
+Per-graph URLs: hit a graph's cluster route from any subcommand by pointing `--uri` at it:
 
 ```bash
 omnigraph read --uri http://server.example.com/graphs/beta --query ./q.gq ...