feat(mcp): immutable canonical_repository_root for cross-repo namespaces (Closes #706)

The MCP server derived the canonical repository root from the install
checkout the server script lives in (PROJECT_ROOT), so any namespace that
ran the server against an external repository (e.g. eagenda-author, or the
mcp-control-plane reviewer/merger namespaces) failed every mutation: the
branches-only / worktree-membership guards (#274) compared the task
workspace against the Gitea-Tools .git directory it can never belong to.

Separate the two concepts:
- PROJECT_ROOT stays the immutable code/install root.
- A new, optional, namespace-scoped canonical_repository_root binds the
  session to the working root of the target repository.

Implementation:
- canonical_repository_root.py: resolve the binding from profile/env
  (GITEA_CANONICAL_REPOSITORY_ROOT overrides the profile field), validate
  existence + git identity, fail closed on missing/conflicting/forged
  bindings. Preserves the single-repo default when unconfigured.
- session_context_binding.py: pin canonical_repository_root into the
  immutable #714 session context; drift fails closed.
- namespace_workspace_binding.py: route the configured target root through
  resolve_namespace_mutation_context so #274 / membership guards evaluate
  against the target repository.
- gitea_mcp_server.py: seed + enforce the binding in the mutation preflight
  (both purity-order and FORCE_PRODUCTION_GUARDS paths); validate repository
  identity and git common-directory membership.
- gitea_config.py: validate the optional absolute-path profile field.
- gitea_auth.py: surface the config-sourced field through get_profile.

No weakening of root-checkout, remote/repository, or anti-stomp guards; the
single-repo Gitea-Tools path is unchanged.

Tests: two distinct real git repositories/worktrees prove cross-repository
bindings resolve, enforce membership, and fail closed on forged/conflicting
identity and simultaneous prgs/mdcps isolation.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

namespace_workspace_binding.py

@@ -109,8 +109,17 @@ def resolve_namespace_mutation_context(
     session_lease_worktree: str | None = None,
     worktree: str | None = None,
     profile_name: str | None = None,
+    configured_canonical_root: str | None = None,
 ) -> dict:
-    """Shared workspace resolution for runtime_context and mutation guards."""
+    """Shared workspace resolution for runtime_context and mutation guards.
+
+    When *configured_canonical_root* is supplied (a cross-repository namespace
+    bound to an external target repository, #706), the canonical repository root
+    is that configured target rather than the MCP install checkout. This keeps
+    the branches-only / worktree-membership guards (#274) evaluating against the
+    repository the namespace actually mutates. Without it the single-repo
+    default is preserved: the canonical root follows the process checkout.
+    """
     demotions: list[str] = []
     workspace, binding_source = resolve_namespace_workspace(
         role_kind=role_kind,
@@ -132,7 +141,11 @@ def resolve_namespace_mutation_context(
         env=env,
         profile_name=profile_name,
     )
-    canonical_root = amw.resolve_canonical_repo_root(process_root, process_root)
+    configured = (configured_canonical_root or "").strip()
+    if configured:
+        canonical_root = os.path.realpath(configured)
+    else:
+        canonical_root = amw.resolve_canonical_repo_root(process_root, process_root)
     return {
         "workspace_path": workspace,
         "workspace_binding_source": binding_source,
@@ -258,6 +271,7 @@ def assess_namespace_mutation_workspace(
     session_lease_worktree: str | None = None,
     profile_name: str | None = None,
     current_branch: str | None = None,
+    configured_canonical_root: str | None = None,
 ) -> dict:
     """Evaluate namespace workspace binding before preflight/mutation."""
     ctx = resolve_namespace_mutation_context(
@@ -268,6 +282,7 @@ def assess_namespace_mutation_workspace(
         env=env,
         session_lease_worktree=session_lease_worktree,
         profile_name=profile_name,
+        configured_canonical_root=configured_canonical_root,
     )
     mutation_workspace = ctx["workspace_path"]
     binding_source = ctx["workspace_binding_source"]