docs: post-merge code-review fixes (Cursor + cubic on PR #60 era)

Two factual mismatches caught during code-grounded re-review:

- docs/architecture.md: "13 cmd families" was stale — the CLI has 17
  Command variants (Version, Embed, Init, Load, Ingest, Branch, Schema,
  Query, Snapshot, Export, Run, Commit, Read, Change, Policy, Optimize,
  Cleanup). Replaced the count with "command families" so the diagram
  doesn't drift again.

- docs/execution.md: the mutation prose said "every mutation runs on a
  fresh __run__<id> branch", which over-claims. mutation.rs:555 short-
  circuits when the target is already a __run__ branch — the assumption
  there is the caller is managing the surrounding run lifecycle. Added a
  one-paragraph caveat noting the exception with the file:line citation.

Both diagrams unchanged; only annotations / counts adjusted.

docs/architecture.md

@@ -225,7 +225,7 @@ Today, indexes are built explicitly via `ensure_indices`. Reads degrade graceful
 flowchart LR
     classDef l2 fill:#e8f4fd,stroke:#1e6aa8,color:#000
 
-    cli[omnigraph CLI<br/>13 cmd families]:::l2
+    cli[omnigraph CLI<br/>command families]:::l2
     srv_in[Axum HTTP<br/>REST + OpenAPI]:::l2
     auth[Bearer auth<br/>SHA-256 hashed tokens]:::l2
     pol[Cedar policy gate<br/>per request]:::l2