chore: add post-push hook to auto-refresh kyberforge plugin cache

Installing kyberforge via the holocron marketplace caches the plugin at a
specific version. After every push, the marketplace clone and plugin cache
need to be refreshed manually — otherwise new skills added since the last
install are invisible until the user runs `claude plugin update` manually.

Adds a post-push git hook that pulls the holocron marketplace clone and
updates the kyberforge cache automatically after every push, eliminating
the manual refresh step.

## Implementation Notes
- `scripts/git-hooks/post-push` is the canonical source; `install.sh` now
  copies all files in `scripts/git-hooks/` into `.git/hooks/` on fresh
  checkouts, making the pattern extensible for future hooks.
- Hook exits 0 on all failures (warns to stderr) — a stale cache refresh
  never blocks a completed push.
- 11 bats tests cover both the hook and the install.sh copy block, using
  mocked binaries and a temp-tree fixture to avoid touching real state.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01P87CiC58Ru2PPWYTeXtjHT

scripts/install.sh

@@ -62,6 +62,18 @@ for adapter in "${SKILL_ADAPTERS[@]}"; do
   fi
 done
 
+# Install git hooks into the current repo checkout
+HOOKS_SRC="$REPO_ROOT/scripts/git-hooks"
+if [[ -d "$HOOKS_SRC" ]]; then
+  for hook_file in "$HOOKS_SRC"/*; do
+    [[ -f "$hook_file" ]] || continue
+    hook_name="$(basename "$hook_file")"
+    dest_hook="$REPO_ROOT/.git/hooks/$hook_name"
+    cp "$hook_file" "$dest_hook"
+    chmod +x "$dest_hook"
+  done
+fi
+
 echo ""
 echo "Deployed:"
 for entry in "${DEPLOY_FILES[@]}" "${DEPLOY_EXECUTABLES[@]}"; do