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]>

gitea_config.py

@@ -502,6 +502,30 @@ def _validate_allowed_repositories(name, raw):
             )
 
 
+def _validate_canonical_repository_root(name, raw):
+    """Validate the optional per-profile canonical repository root (#706).
+
+    ``canonical_repository_root`` binds a cross-repository namespace to the
+    working root of its target repository (separate from the immutable
+    Gitea-Tools install checkout). It is an absolute filesystem path; existence
+    and git identity are validated at bind time by the runtime guard, not here
+    (config validation stays filesystem-independent). Absent means the
+    single-repo default and is allowed.
+    """
+    if raw is None:
+        return
+    if not isinstance(raw, str) or not raw.strip():
+        raise ConfigError(
+            f"profile '{name}' canonical_repository_root must be a non-empty "
+            "absolute path string to the target repository working root"
+        )
+    if not os.path.isabs(raw.strip()):
+        raise ConfigError(
+            f"profile '{name}' canonical_repository_root {raw!r} must be an "
+            "absolute path"
+        )
+
+
 def _reject_inline_secrets(kind, name, obj):
     for key in _INLINE_SECRET_KEYS:
         if key in obj:
@@ -577,6 +601,9 @@ def _load_v2_contexts(data, path):
         if not isinstance(allowed, list) or not isinstance(forbidden, list):
             raise ConfigError(f"profile '{name}' operation fields must be lists")
         _validate_allowed_repositories(name, raw.get("allowed_repositories"))
+        _validate_canonical_repository_root(
+            name, raw.get("canonical_repository_root")
+        )
         allowed_n = {_normalize_op("gitea", op, name) for op in allowed}
         forbidden_n = {_normalize_op("gitea", op, name) for op in forbidden}
         # Reviewer-identity deadlock rule (#100/#103) applies here unchanged.