Warn when preserved pre-commit hooks do not dispatch pre-commit.d

[quick-ricon]

Context

modules#49 made modules install-hooks restore the full hook set: pre-commit dispatcher, pre-commit.d/gitmodules-guard, pre-commit.d/manifest-encryption, stale hook cleanup, and merge driver repair.

The command intentionally preserves an existing executable .git/hooks/pre-commit rather than overwriting user hooks. That matches prior setup behavior, but it leaves a sharp edge: if the existing hook does not dispatch .git/hooks/pre-commit.d/*, then install-hooks can restore the guard files while they still never run.

Proposed behavior

When modules setup or modules install-hooks sees an existing executable pre-commit that it will preserve, warn if it does not appear to dispatch pre-commit.d.

Possible first heuristic:

• if .git/hooks/pre-commit exists and is executable;
• and its content does not mention pre-commit.d;
• print a warning explaining that modules guards were installed but may not run until the existing hook dispatches that directory.

Do not overwrite the custom hook automatically.

Acceptance criteria

• Existing custom dispatchers that invoke pre-commit.d remain quiet and preserved.
• Existing custom hooks that do not mention/dispatch pre-commit.d produce an actionable warning.
• Tests cover both cases.
• The warning works from normal repos and linked worktrees.

Follow-up shape

This could land either in install-hooks directly or later as part of a modules doctor; the immediate user-risk is that install-hooks now sounds complete, so a warning in the repair command may be most helpful.