Provide Hook for manuall compact

[dsrednicki]

Extend the PreCompact hook to also fire when the user manually runs /compact, not only during auto-compact triggered by context window l imits.

Current behavior: PreCompact only fires during automatic context compaction (when the context window limit is hit).

Desired behavior: PreCompact fires on both auto-compact and manual /compact.

Why it matters: Any hook registered on PreCompact is silently skipped when users compact manually, making the hook unreliable for use cases like saving state, logging, or reminders before context is lost.

Suggestion: Optionally pass a flag to distinguish trigger source (e.g. COMPACT_TRIGGER=manual vs COMPACT_TRIGGER=auto) so hooks can handle both cases differently if needed.