Add ntpd/clock-sync check and a pre-commit hook for exec bits

checks/ntpd.sh reports whether ntpd is running and whether the clock's
offset is within tolerance (ok <50ms, warn <200ms, down beyond that or
if ntpq doesn't respond) - clock drift is a silent failure that
otherwise only surfaces later as TLS handshake failures or misleading
cross-jail log timestamps.

Also add .githooks/pre-commit + core.hooksPath, since this checkout is
on Windows where core.fileMode is false (the filesystem doesn't
reliably preserve the executable bit) - without it, a plain `git add`
on a new check script silently stages it as non-executable, and
render.sh skips non-executable files with no visible error.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

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