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.

tests/test_mcp_server.py

@@ -3879,7 +3879,7 @@ class TestPreflightVerification(unittest.TestCase):
         os.environ["GITEA_TEST_PORCELAIN"] = " M reviewer_edit.py\n"
         with self.assertRaises(RuntimeError) as ctx:
             mcp_server.verify_preflight_purity()
-        self.assertIn("Reviewer profile is forbidden from modifying tracked workspace files", str(ctx.exception))
+        self.assertIn("forbidden from modifying tracked workspace files", str(ctx.exception))
         self.assertIn("reviewer_edit.py", str(ctx.exception))
 
         # Foreign pre-existing dirty state does not block when unchanged.
@@ -3945,7 +3945,8 @@ class TestPreflightVerification(unittest.TestCase):
         with self.assertRaises(RuntimeError) as ctx:
             mcp_server.verify_preflight_purity(worktree_path=worktree)
         msg = str(ctx.exception)
-        self.assertIn("active task workspace root", msg)
-        self.assertIn("inspected git root", msg)
-        self.assertIn("dirty files: task_file.py", msg)
-        self.assertIn("dirty scope:", msg)
+        self.assertIn("resolved workspace", msg)
+        self.assertIn(worktree, msg)
+        self.assertIn("worktree_path argument", msg)
+        self.assertIn("task_file.py", msg)
+        self.assertIn("author namespace", msg)