fix(cache-timer): reset the countdown only when the main conversation was compacted

[arasovic]

What changes

• cache-timer: the countdown now clears only after the main conversation was really compacted. A subagent's compaction, a skipped compaction and a summary prepared ahead of time (precompute) leave it running. The hook waits for the compaction's result before it decides. Version 0.1.0 → 0.1.1.
• compact-keeper: tests only. A new test checks that no handoff is written for a skipped, a precompute or a subagent compaction, and that a main-thread compaction writes one.

Why

Claude Code's idle compaction fires session.compact with trigger: 'auto', so cache-timer clears on it as intended. The old hook also cleared before next(e), so it reset on skips, precomputes and subagent compactions, which do not touch the main thread's cache.

Verification

• claude plugin test and claude plugin validate --strict for every mod, plus the marketplace: pass.
• ./typecheck.sh: pass.
• Each guard removed in turn makes its test fail (cache-timer: agentId, skip, precompute; compact-keeper: skip, precompute, agentId).
• Not checked in a live session.