fix(migration): remediate 6 MAJOR + 3 MINOR trekreview findings + stale rename test

MAJOR
- 9e97cd5 40-validate-standalone.sh: route a target's sc2_gate to its dedicated
  gate (config-audit → 50-config-audit-sc2.sh), mirroring 99-dryrun.sh, so --all
  no longer falsely FAILs config-audit on the machine-locked v5.0.0 tests.
- 1708e90 99-dryrun.sh: assert EXACTLY one tag survives (F5); a partial tag-strip
  no longer silently reports the wrong tag via head -1.
- 4e494c8 99-dryrun.sh: capture the SC2 standalone failing set from the dry-run's
  own prepped extract ($dest), not the 40-validate side-effect clean room.
- aeb6292 00-preflight.sh: assert every map path is whitespace/glob-free, making
  the word-split path handling in 99-dryrun.sh sound.
- 5d112cb extract the SC6 DROP + SC2 regression detectors into sc6-check.sh /
  sc2-regression.sh and add sc-checks.test.mjs — a negative test proving each
  detector FIRES (force-fresh re-extraction would undo a planted file-drop).
- 9e588ca 10-extract.sh re-asserts git filter-repo before use (self-heal runs
  preflight only on a missing mirror); RUNBOOK lists git-filter-repo + python3>=3.6.

MINOR
- bc0f8a7 plugin-map.json: reset ms-ai-architect blob_strip_safe to null
  (00-preflight.sh populates it per run).
- 8d649e9 99-dryrun.sh: gate SC6 behind extract success; a failed extract is
  labelled (extract failed), not a content DROP.
- 4044c49 99-dryrun.sh: guard mktemp — an empty capture is an error, not a
  false zero-regression PASS.

Also: 00-preflight.test.mjs asserted all 3 'renamed' plugins carry >=2 paths, but
llm-security became single-path in 836b8e9 (copilot was a coexisting plugin, not a
rename) — a stale pre-existing failure. Aligned the test to the ratified map and
added a positive single-path lock against re-introducing the 87-file-drop defect.

Verified: full dry-run 11/11, 0 pushes; sc-checks/99-dryrun/40-validate/00-preflight/
60-rewrite suites green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/marketplace-polyrepo-migration/migration/plugin-map.json

@@ -93,7 +93,7 @@
       "repo_url": "https://git.fromaitochitta.com/open/ms-ai-architect.git",
       "has_vendor_ds": true,
       "blob_strip": true,
-      "blob_strip_safe": true,
+      "blob_strip_safe": null,
       "test_cmd": "bash tests/validate-plugin.sh",
       "standalone_caveat": "148 MB screenshot blob-bomb (F3): single filter-repo pass strips >1MB blobs when blob_strip_safe (all under playground/screenshots/), else surgical --path-glob; vendors the design-system"
     },