add pre-push hook plumbing under .githooks/

`make hooks` already sets `core.hooksPath=.githooks/`, and the
pre-push gate at `.githooks/pre-push` already runs `make push-check`
— but the pql-installed pre-commit and post-merge shims live at
`.git/hooks/`, which take precedence and silently disable .githooks/.

Add the missing pre-commit / post-merge shims under .githooks/ so
`make hooks` becomes a single-step install: pre-push enforcement,
pql planning-state auto-export on commit, and auto-import on pull
all fire from the canonical .githooks/ location.

Co-Authored-By: Claude <noreply@anthropic.com>

.githooks/pre-commit

@@ -0,0 +1,3 @@
+#!/bin/sh
+# pql: source .pql/hooks/pre-commit (planning-state auto-export)
+. "$(git rev-parse --show-toplevel)/.pql/hooks/pre-commit"