chore: remove stale chunk/skills references from vision, scripts, and tests

## Why
The repo moved from a chunk-based delivery model with `.agents/skills/`
as the canonical skill source to a plugin model. Several files retained
references to the old model that were either dead code or misleading
framing.

## Impact
- `tests/test-install.sh`: dead `.agents/skills/` test blocks removed;
  suite now tests only what `install.sh` actually deploys
- `scripts/deploy-manifest.sh`: `DEPLOY_SKILLS_SRC` variable and stale
  `sync.sh (Chunk 6)` comment removed
- `tests/test-git-hooks-install.sh`: fixture stub no longer declares
  the removed `DEPLOY_SKILLS_SRC` variable
- `docs/VISION.md`: chunk delivery framing replaced with plugin model
  language throughout; manual test plan date updated
- `tests/test-instructions-and-docs.sh`: stale test plan date flagged
  as pre-refactor so readers know a re-run is needed

Refs: #15

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test-instructions-and-docs.sh

@@ -284,7 +284,7 @@ echo "────────────────────────
 echo "MANUAL TEST PLAN (run in a fresh Claude session)"
 echo "─────────────────────────────────────────────────────"
 echo ""
-echo "Results last run: 2026-05-17 (multiple rounds)"
+echo "Results last run: 2026-06-28 (pre-plugin-refactor; re-run needed)"
 echo "  PASS: 1, 2, 3, 5, 6, 7, 8, 9"
 echo "  INCONCLUSIVE: 4 (no remote configured in test environment)"
 echo ""