test(snapshots): make byte/snapshot tests hermetic + re-seed baseline

The COL collision-scanner and the CLAUDE.md cascade resolve ~/.claude from
process.env.HOME (active-config-reader). Snapshot/byte CLIs were spawned with
the developer's real HOME, so they picked up installed plugins/skills and the
user CLAUDE.md — making the v5.0.0 + default-output snapshots machine- and
time-dependent. They were seeded 2026-05-01 with COL=1 (a real ~/.claude skill
collision) and drifted to COL=0 after the polyrepo split: 26 pre-existing
failures unrelated to Batch 1.

Fix (test-only, no production change):
- tests/helpers/hermetic-home.mjs — empty temp HOME, mirroring the pattern
  collision.test.mjs already uses for the COL unit test.
- 7 harnesses spawn CLIs (or call lint()) under the hermetic HOME, so output
  depends only on committed fixtures. Determinism verified across runs.
- Re-seeded all snapshots under hermetic HOME via SEED_SNAPSHOT/UPDATE_SNAPSHOT
  (added a SEED guard to the frozen v5.0.0 byte tests). Snapshots now reflect
  the fixture alone (COL=0, fixture-only activeConfig counts).
- Also re-seeded the unused env-aware snapshots (manifest/whats-active/
  plugin-health), which had baked dozens of real ~/.claude skill/plugin names
  into the committed repo — privacy cleanup.

Full suite: 812/812 green, stable across 3 runs.

tests/snapshots/v5.0.0/plugin-health.json

@@ -1,14 +1,28 @@
 {
   "scanner": "PLH",
   "status": "ok",
-  "files_scanned": 1,
-  "duration_ms": 17,
-  "findings": [],
+  "files_scanned": 0,
+  "duration_ms": 2,
+  "findings": [
+    {
+      "id": "CA-PLH-001",
+      "scanner": "PLH",
+      "severity": "info",
+      "title": "No plugins found",
+      "description": "No Claude Code plugins found under tests/fixtures/marketplace-medium",
+      "file": null,
+      "line": null,
+      "evidence": null,
+      "category": null,
+      "recommendation": "Ensure plugins have .claude-plugin/plugin.json",
+      "autoFixable": false
+    }
+  ],
   "counts": {
     "critical": 0,
     "high": 0,
     "medium": 0,
     "low": 0,
-    "info": 0
+    "info": 1
   }
 }