fix: summarize conversations only once idle, and refresh summaries as they grow

[minyek]

Description

Summaries are write-once. A conversation that gets summarized while still in
progress keeps a summary of only its early portion forever — every later turn
is invisible to the summary. For long-running or resumed conversations, the
one-line summary shown next to search results is therefore misleading.

The fix has two complementary halves: defer summarizing a conversation until
it has gone idle (so a still-active conversation is never frozen into a partial
snapshot), and refresh a summary when its transcript later grows. Note the
behavioral change from the defer half: a conversation is no longer summarized
until it has been idle for the quiescence window (EPISODIC_MEMORY_SUMMARY_QUIESCENCE_HOURS,
default 1h), so very recent conversations intentionally have no summary yet.

(Search results themselves were always complete — search runs over indexed
exchanges, kept fresh via INSERT OR REPLACE. This fixes only the summary
displayed alongside them.)

Symptom

There is no error in the logs — the symptom is silently wrong output:

• <archive>/<project>/<session>-summary.txt for an active or long conversation
  describes only the first handful of exchanges.
• Continuing the conversation over hours/days never updates it.
• stats/verify count the file as "summarized", so it is never revisited.

Environment

• episodic-memory v1.4.2
• Claude Agent SDK summarizer
• All installs affected; most visible to anyone who keeps long or resumed
  sessions.

Steps to Reproduce

1. Start a conversation and let a sync summarize it early (a few exchanges).
2. Continue the conversation for many more turns over time.
3. Run sync again (or let the SessionStart background sync run).
4. Inspect <archive>/<project>/<session>-summary.txt.
5. Observe it still summarizes only the early portion — never re-summarized,
   even though the transcript has grown substantially.

Root Cause

shouldQueueForSummary only re-queued a conversation when its sentinel was
missing or a stale error marker. Once a real summary existed, the
conversation was treated as done permanently:

1. No coverage signal. Nothing recorded how much of the transcript a given
   summary reflected, so transcript growth was undetectable.
2. Append-only transcripts. A summary written at exchange 5 is never
   refreshed at exchange 500 — the file only ever grows, but nothing acted on
   that growth.
3. No idle awareness. Even if growth had been detected, summarizing an
   in-progress conversation would freeze a mid-session snapshot that is itself
   immediately stale.

Fix Summary

1. Coverage header. Real summaries now carry a machine-readable first line —
   __COVERAGE__ {"bytes":<n>,"lastExchange":"<iso>","schema":1} — recording the
   archived JSONL byte size the summary reflects (formatSummaryFile /
   parseSummaryFile, buildCoverage, writeSummary).
2. Growth-driven re-queue. shouldQueueForSummary(path, currentArchiveBytes)
   re-queues a real summary when the archive has grown past its covered bytes
   (append-only ⇒ a size increase means new content).
3. Quiescence gate. isQuiescent + EPISODIC_MEMORY_SUMMARY_QUIESCENCE_HOURS
   (default 1) only (re)summarize after the conversation has been idle for the
   window, so a mid-session snapshot is never frozen. Idle is measured from the
   last exchange's timestamp — the archive copy's mtime is reset on every
   sync and is unreliable.
4. Gentle upgrade. Legacy header-less summaries are stamped with a
   current-size baseline (ensureCoverageBaseline) — a header rewrite, no LLM
   call — so existing installs gain a growth baseline without a mass
   re-summarization on upgrade.
5. Preserve on failure. A re-summary failure now keeps the prior real
   summary; an error sentinel is written only for a first-ever failure, so a
   transient SDK error never discards a good summary (writeErrorSentinelIfNew).
6. Display. search strips the coverage header from the summary shown
   alongside results.

Internally, the gate + write + error-sentinel logic is now shared
(needsSummary, writeErrorSentinelIfNew, summarizeIfQuiescent) across the
three indexer paths and the sync loop, and the _HOURS config defaults convert
to milliseconds at a single boundary (MS_PER_HOUR).

Tests added

• test/summary-coverage.test.ts — coverage header round-trip and legacy baseline stamping.
• test/summary-quiescence.test.ts — quiescence verdicts and _HOURS env parsing.
• test/summary-sentinel.test.ts — growth-aware gate decision table; needsSummary / writeErrorSentinelIfNew.
• test/sync-resummary.test.ts — growth-driven re-summary, no-op when unchanged, preserve-on-failure.
• test/search-summary-strip.test.ts — the header never leaks into the displayed summary.
• Updates to sync-error-sentinel, show, and sync tests for the new header/gate.

Fix Verified

• Full suite: 242 tests across 40 files pass.
• Real end-to-end on the built dist (isolated temp dirs, real summarizer):
  • Seed an 8 237-byte conversation → first sync writes a summary stamped "bytes":8237.
  • Grow the transcript to 41 069 bytes → next sync re-summarizes; coverage re-stamped "bytes":41069.
  • No further growth → next sync performs no re-summary.

Caveat / Notes

• Re-summary is triggered by byte growth, not an exchange-count delta. This
  is deliberate: transcripts are append-only and the per-sync coverage scan is
  ~23 ms even against a multi-GB archive, so no recency cutoff or minimum-delta
  threshold is needed. The quiescence gate bounds work to one re-summary per
  idle-after-growth episode.