refactor: remove deprecated devx.ci.discover_runners wrapper

All workflows now use devx.molecule.discover_runners directly. The
deprecated wrapper (added in Phase 1c for backward compatibility) is
no longer referenced by any workflow in devx, grm, or infra.

- Delete src/devx/ci/discover_runners.py
- Remove `devx ci discover-runners` CLI subcommand
- Update tests to import from devx.molecule.discover_runners
- Remove TestDeprecationWrapper class
- Update docs, README, AGENTS.md, ADR-0002

DEVX-160

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

tests/unit/test_cli.py

@@ -73,13 +73,6 @@ class TestCiCommands:
         assert result.exit_code == 0
         mock_run.assert_called_once_with("devx.ci.detect_release_commit", [])
 
-    @patch("devx.cli._run_module")
-    def test_ci_discover_runners(self, mock_run: MagicMock) -> None:
-        runner = CliRunner()
-        result = runner.invoke(cli, ["ci", "discover-runners", "positional"])
-        assert result.exit_code == 0
-        mock_run.assert_called_once_with("devx.ci.discover_runners", ["positional"])
-
     @patch("devx.cli._run_module")
     def test_ci_doc_coverage(self, mock_run: MagicMock) -> None:
         runner = CliRunner()