refactor(engine): remove the legacy __run__ branch guard (MR-770)

With the v2→v3 migration sweeping stale `__run__*` branches off `__manifest`
on first read-write open, the defense-in-depth `is_internal_run_branch` guard
is no longer needed.

- delete `db/run_registry.rs`; drop the module + re-export from `db/mod.rs`
- collapse `is_internal_system_branch` to the schema-apply-lock check only
- `ensure_public_branch_ref`: drop the run-ref rejection; `__run__*` is now an
  ordinary branch name
- `branch_merge`: reject `is_internal_system_branch` (was run-only) so the
  schema-apply lock is rejected consistently with create/delete — a small,
  deliberate tightening
- update the inline schema-apply test + the writes integration tests
  (`public_branch_apis_reject_internal_run_refs` →
  `public_branch_apis_reject_internal_system_refs`, which also asserts
  `__run__*` now creates successfully)
- docs: flip the "pending production sweep / defense-in-depth" notes to
  "auto-swept by the v2→v3 migration"; document the read-only-open limitation

Known residual: the inert `_graph_runs.lance` / `_graph_run_actors.lance` bytes
remain until a `StorageAdapter::delete_prefix` primitive lands.

docs/dev/writes.md

@@ -14,8 +14,11 @@ publisher's row-level CAS on `__manifest` is the single fence.
 
 - No `RunRecord`, no `_graph_runs.lance`, no `_graph_run_actors.lance`.
 - No `omnigraph run *` CLI subcommands and no `/runs/*` HTTP endpoints.
-- No `__run__<id>` staging branches. (Legacy on-disk artifacts from
-  pre-MR-771 repos are inert; MR-770 sweeps them in production.)
+- No `__run__<id>` staging branches; `__run__*` is no longer a reserved
+  name. The branch-name guard was removed in MR-770, and any stale
+  `__run__*` branch on an upgraded graph is swept off `__manifest` by the
+  v2→v3 internal-schema migration on first read-write open. (The inert
+  `_graph_runs.lance` bytes remain until a `delete_prefix` primitive lands.)
 - Cancelled mutation futures leave **no graph-level state** — only orphaned
   Lance fragments, which the existing `omnigraph cleanup` pipe reclaims.