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-git-hooks-install.sh

@@ -32,7 +32,6 @@ cat > "$TEMP_REPO/scripts/deploy-manifest.sh" << 'EOF'
 DEPLOY_FILES=()
 DEPLOY_EXECUTABLES=()
 DEPLOY_DIRS=()
-DEPLOY_SKILLS_SRC=".agents/skills"
 EOF
 
 # Copy the real install.sh and git-hooks into the temp repo