fix(design-detect): audit directories scan as dumps; scans print probe lines on stderr; refused base always exits 1; PATH loses project entries

Adversarial review (Claude subagent):

- A DIRECTORY target under designs/ (the audit dir, which the prose hands the
  agent as REPORT_DIR) classified as an artifact, so the engine walked its
  dom/ subtree WITH inline ignores honored. Any directory under designs/ is
  now scanned as dumps.
- A scan whose probe no longer finds an engine wrote its sentinel lines to
  stdout and exited 0, so `scan > "$_DJ"` captured "IMPECCABLE_NOT_AVAILABLE"
  as the scan result and the rendered bash read a clean scan. Probe lines go
  to stderr on every path; stdout is the JSON document or nothing.
- A refused --changed base exited 0/2 when explicit targets were also given;
  it folds into the exit code (1 over 2 over 0). A trailing --changed no
  longer defaults to main.
- A hand-edited `design_detector: Off` re-enabled the detector; the value is
  compared case-insensitively.
- The engine inherited PATH entries inside the project (a direnv .envrc
  adding node_modules/.bin); those are filtered like every other project path.
- DOM_DUMP_MISSING names the case where the dump script wrote nothing.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

lib/design-detect-contract.ts

@@ -46,6 +46,7 @@ export const SENTINEL = {
   DETECT_JSON: 'DETECT_JSON',
   /** printed by rendered bash after a DOM dump is persisted */
   DOM_DUMP_OK: 'DOM_DUMP_OK',
+  DOM_DUMP_MISSING: 'DOM_DUMP_MISSING',
   DOM_DUMP_REDACTION_BLOCKED: 'DOM_DUMP_REDACTION_BLOCKED',
   DOM_DUMP_TOO_LARGE: 'DOM_DUMP_TOO_LARGE',
   DESIGN_MD_FORMAT: 'DESIGN_MD_FORMAT',
@@ -75,7 +76,7 @@ export const SELF_DESCRIBING_SENTINELS: readonly string[] = [
   SENTINEL.ENGINE_UNTESTED, SENTINEL.DETECT_EXIT, SENTINEL.DETECT_REFUSED, SENTINEL.DETECT_NO_TARGETS,
   SENTINEL.DETECT_TIMEOUT, SENTINEL.DETECT_PARSE_ERROR, SENTINEL.DETECT_OUTPUT_TOO_LARGE,
   SENTINEL.DESIGN_MD_TOKEN_REF_INVALID, SENTINEL.DESIGN_MD_WRITTEN, SENTINEL.DESIGN_MD_BACKUP, SENTINEL.DESIGN_MD_EDIT_REFUSED,
-  SENTINEL.PROBE_STEP, SENTINEL.ENGINE_STDERR,
+  SENTINEL.PROBE_STEP, SENTINEL.ENGINE_STDERR, SENTINEL.DOM_DUMP_MISSING,
 ];
 
 /** Engine versions the committed fixtures were captured from. */