post-checkout hook: guard with if + force exit 0 (worktree-safe)

The pql post-checkout hook is untracked (local `pql init` install), so a fresh
`git worktree add` has none — and `[ -f x ] && . x` returns 1 when absent (the
script's last statement), which worktree add propagates as a hard failure.
Use an if-guard and always exit 0: post-checkout is best-effort and must never
abort a checkout / worktree creation. (Worth reporting upstream to pql.)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.githooks/post-checkout

@@ -1,6 +1,9 @@
 #!/bin/sh
 # pql: source .pql/hooks/post-checkout (rebuild pql.db on branch checkout).
-# Guard on existence so a fresh `git worktree add` — whose toplevel has no
-# .pql/hooks yet — doesn't abort the checkout (the hook is best-effort).
+# The pql hook is untracked (a local `pql init` install), so a fresh
+# `git worktree add` has no .pql/hooks — source it only when present, and
+# always exit 0: post-checkout is best-effort and must never abort the
+# checkout / worktree creation.
 hook="$(git rev-parse --show-toplevel)/.pql/hooks/post-checkout"
-[ -f "$hook" ] && . "$hook"
+if [ -f "$hook" ]; then . "$hook"; fi
+exit 0