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/git-hooks/post-push

@@ -0,0 +1,17 @@
+#!/usr/bin/env bash
+# Refresh the kyberforge plugin cache after every push
+set -euo pipefail
+
+echo "→ Refreshing holocron marketplace clone..."
+if git -C ~/.claude/plugins/marketplaces/holocron pull --quiet; then
+  echo "→ Updating kyberforge plugin cache..."
+  if claude plugin update kyberforge; then
+    echo "✓ kyberforge cache updated"
+  else
+    echo "⚠ claude plugin update kyberforge failed — run it manually" >&2
+  fi
+else
+  echo "⚠ Failed to pull holocron marketplace clone — run 'git -C ~/.claude/plugins/marketplaces/holocron pull' manually" >&2
+fi
+
+exit 0