W7: Possibilities matrix (work-already-done fast path) (#522)

* [W7] Add possibilities_matrix_enabled feature flag (default off)

* [W7] Add _work_appears_done predicate (status+commits+PR+ACs+no-open-findings)

* [W7] Add CI-green quality proxy for the fast path (local fallback on no-CI)

* [W7] Add work-already-done fast path in i_am_done (slimmed gates, no rich plan)

* [W7] Add WORK_ALREADY_DONE prompt state

* [W7] Make fast path mypy-clean (cast to helpers for _resolve_ci_status; typed mock locals)

* [W7] Extract _all_criteria_addressed to bring _work_appears_done under xenon B

---------

Co-authored-by: Renn F <rennf93@users.noreply.github.com>

tests/unit/config/test_possibilities_matrix_flag.py

@@ -0,0 +1,27 @@
+"""Possibilities matrix (work-already-done fast path) is gated by a default-off
+config flag, registered as a panel-tunable feature flag (the #487 lesson)."""
+
+from __future__ import annotations
+
+import os
+from unittest import mock
+
+from roboco.config import Settings
+from roboco.services.settings import FEATURE_FLAGS, validate_setting
+
+
+def test_possibilities_matrix_disabled_by_default() -> None:
+    assert Settings().possibilities_matrix_enabled is False
+
+
+def test_possibilities_matrix_reads_env_var() -> None:
+    with mock.patch.dict(os.environ, {"ROBOCO_POSSIBILITIES_MATRIX_ENABLED": "true"}):
+        assert Settings().possibilities_matrix_enabled is True
+
+
+def test_possibilities_matrix_flag_registered_in_feature_flags() -> None:
+    assert "possibilities_matrix_enabled" in [key for key, _ in FEATURE_FLAGS]
+
+
+def test_possibilities_matrix_flag_validates_as_bool() -> None:
+    validate_setting("possibilities_matrix_enabled", "true")