Hooks: markers/ leaks hook-dedup-* files (134k+) — dedup markers never pruned

[magicpro97]

Summary

hooks/hook_runner.py _check_and_set_dedup() writes one marker file hook-dedup-{event}-{payload_hash} per invocation to suppress double-fired hooks within a 500 ms window. The payload hash is a per-call MD5, so every unique tool call creates a new file that is never deleted.

Impact (Facts)

• Observed ~/.copilot/markers/ containing 134,426 hook-dedup-* files (ls | grep -c hook-dedup).
• Every hook invocation does mkdir + write_text into that directory; a directory with 130k+ entries slows all hook events (sessionStart/preToolUse/postToolUse/...).

Root cause

The 500 ms dedup window only needs a marker for a fraction of a second, but nothing ever removes stale markers.

Fix

Add a time-gated _prune_stale_dedup_markers() that removes hook-dedup-* files older than a 5 s TTL, at most once per 60 s (stamp file dedup-sweep.stamp), fully fail-open.

Acceptance criteria

• Stale hook-dedup-* markers are pruned automatically.
• Fresh marker for the current call survives (dedup still works within 500 ms).
• Regression test added.
• tests/test_hook_runner_entrypoints.py passes.