fix(crisp): re-arm stale check on customer messages only; dedupe skips threads with no recent customer message

[lihsaa591]

Why

Two support-reported symptoms, each diagnosed from live Crisp metadata (message types, roles and timestamps only; no content).

session_3931e910 was re-investigated 7 times in 3 weeks. Three of those (Sep 28, Oct 9, Oct 13) were triggered by a support agent's reply, not the customer. Both that finished returned no_defect and still posted another note. The stale path treated any text message as new activity.

session_f9e20b9b was matched to everest-forms-pro#1245 two months after support resolved it (Aug 14). The state:resolved event is real, but Crisp had the thread as unresolved again, with no new message or event, from at least the Sep 2 seed (commit 3be1fa7). The bot never touched it before Oct 13, so it did not reopen it. crisp-dedupe-active.mjs had no freshness gate and matched it anyway.

Also found: active.last is absent (not frozen) on email-origin conversations, so active.last ?? created_at measured age-since-creation. That is why every "stale NNNh" log line counted back to the creation date.

Changes

• crisp-classify.mjs: the stale path re-arms only on a new customer message (shouldReclassifyStale). checkedThroughAt never moves backwards (nextCheckedThroughAt).
• crisp-classify.mjs: the pre-fetch gate takes the newest of active.last / updated_at / created_at as a hint only. Staleness is recomputed from real message timestamps once messages are fetched.
• crisp-dedupe-active.mjs: skips conversations whose newest customer message is older than 14 days (DEDUPE_MAX_QUIET_DAYS). Skipped threads are not marked notified, so a new customer message re-evaluates them. The step summary gains a "Skipped" count.
• New pure helpers and tests in scripts/crisp-activity.mjs / crisp-activity.test.mjs.
• CHANGELOG entry, and debug-crisp-triage-not-investigated skill updated (it said active.last was "frozen", which was wrong).

Not changed

• Reopen path. A reopen is itself a signal and its delta still includes agent text.
• seed-escalated.mjs. It stamps from all message types, which only makes a thread re-arm later.
• The unresolved-state flip itself. Its cause isn't visible from the API. Support can check the conversation's activity history in Crisp.

Behavior change to be aware of

The 30-day AUTO_ESCALATE_MAX_HOURS cap is now measured from the last real message, not from creation. A long-lived thread that is still active can therefore become eligible, where before it was excluded by age. I expect this to be rare: conversations active at onboarding were seeded, and ones created later were already processed while under 30 days old.

Testing

• node --test scripts/*.test.mjs: 73 pass, including 13 new tests. Cases include the real shapes of both sessions (roles and timestamps only).
• npm test fails with MODULE_NOT_FOUND on a clean master too (Node 22 and the directory arguments), so it was already broken locally and is unrelated to this change.
• Not tested: the changed main() paths against live Crisp, since they aren't unit-testable as written.
• After merge, watch the next few runs' stale ...h log lines and the dedupe step summary's "Skipped" count.