fix(mcp): invalidate review session state on cross-profile activation (Closes #690)

A mid-run profile switch (reviewer -> author -> reviewer) left workflow-load
proof, reviewer lease binding, the review decision lock, live namespace
health, and preflight identity/capability stamps intact, so a formal verdict
could be recorded under contaminated session state.

- gitea_activate_profile now invalidates all review-critical session state
  on a cross-profile switch, in memory and in durable state keyed by either
  profile identity, and reports the invalidation + re-preflight requirement.
- Full reviewer preflight (whoami, load_review_workflow,
  resolve_task_capability(review_pr), head re-pin, lease re-acquire) is
  required before any formal verdict after a switch; switching back cannot
  resurrect the stale run.
- Namespace provenance: optional launcher-declared GITEA_MCP_NAMESPACE is
  reported by whoami/runtime context/capability resolution, and a declared
  namespace that disagrees with a task's required namespace fails closed.
- Docs: supported pattern is separate session/namespace per role, not
  in-process profile hopping mid-review.

docs/mcp-namespace-health.md

@@ -86,3 +86,24 @@ When a namespace returns EOF, follow
 
 When blocked, repair the IDE namespace and re-record a healthy
 `client_namespace` assessment before retrying the mutation.
+
+## Namespace provenance (#690)
+
+A server process cannot derive the name of the client-managed namespace it is
+registered under, so the launcher may declare it with the
+`GITEA_MCP_NAMESPACE` environment variable (e.g. `GITEA_MCP_NAMESPACE=gitea-reviewer`).
+
+- `gitea_whoami`, `gitea_get_runtime_context`, and
+  `gitea_resolve_task_capability` report `namespace_provenance`: the declared
+  client namespace, the active execution profile, and — for tasks with a
+  required namespace (`review_pr`/`submit_review` → `gitea-reviewer`,
+  `merge_pr` → `gitea-merger`) — a `mismatch` verdict.
+- A declared namespace that disagrees with the requested task's required
+  namespace **fails closed** (`allowed_in_current_session=false` with a STOP
+  guidance entry).
+- An undeclared namespace is reported as `namespace_source="unknown"` and is
+  never treated as proof either way.
+- A profile switch via `gitea_activate_profile` clears all recorded live
+  namespace-health assessments; re-probe through the client before further
+  review/merge mutations.
+