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.

tests/conftest.py

@@ -41,6 +41,7 @@ def _reset_mutation_authority(monkeypatch):
         "GITEA_REVIEWER_WORKTREE",
         "GITEA_MERGER_WORKTREE",
         "GITEA_RECONCILER_WORKTREE",
+        "GITEA_MCP_NAMESPACE",
     ]:
         monkeypatch.delenv(env_key, raising=False)
 
@@ -115,6 +116,7 @@ def _reset_mutation_authority(monkeypatch):
         monkeypatch.setattr(mcp_server, "_ACTOR_IDENTITY_CACHE", {})
     monkeypatch.setattr(mcp_server, "_REVIEW_DECISION_LOCK", None)
     monkeypatch.setattr(mcp_server, "_LIVE_NAMESPACE_HEALTH", {})
+    monkeypatch.setattr(mcp_server, "_PROFILE_SWITCH_INVALIDATION", None)
     monkeypatch.setattr(mcp_server, "_preflight_whoami_called", False)
     monkeypatch.setattr(mcp_server, "_preflight_capability_called", False)
     monkeypatch.setattr(mcp_server, "_preflight_resolved_role", None)