feat: isolate MCP workspace binding across role namespaces (Closes #510)

Namespace-scoped workspace resolution prevents GITEA_AUTHOR_WORKTREE from
poisoning reviewer, merger, and reconciler purity checks. Each role uses
its own worktree env var with actionable binding-source errors and safe
reconnect guidance. Preserves #274/#475 author and control-checkout guards.

author_mutation_worktree.py

@@ -12,6 +12,8 @@ import subprocess
 BASE_BRANCHES = frozenset({"master", "main", "dev"})
 ACTIVE_WORKTREE_ENV = "GITEA_ACTIVE_WORKTREE"
 AUTHOR_WORKTREE_ENV = "GITEA_AUTHOR_WORKTREE"
+# Author-only: reviewer/merger/reconciler namespaces use role-specific env vars
+# via namespace_workspace_binding (#510).
 
 
 def _normalize_path(path: str) -> str: