fix: harden post-compaction continuation guard (supersedes #61)

[willytop8]

Summary

Follow-up to #61 (and #58 before it) hardening the post-compaction goal
continuation guard. This branch keeps @harryzhou2000's epoch-guard work from #61
as its base — his commit is preserved — and corrects three issues that only
surface against the event shapes OpenCode actually emits.

The event shapes were confirmed against the vendored @opencode-ai/sdk
type definitions rather than inferred from test fixtures:

• EventMessageUpdated is { type, properties: { sessionID, info } } — it never
  carries parts. Parts arrive on message.part.updated, which this plugin does
  not observe.
• EventSessionCompacted is { type, properties: { sessionID } } — no id, no
  compactionID/summaryID/messageID, and no sync variant in the plugin-facing
  Event union.

What changes

1. Identity-less compaction re-delivery no longer trips the breaker. Because a
   real session.compacted has no identity field, the identity-based dedup from
   fix: guard goal continuation across compaction epochs #61 can never fire against a live host, so a single duplicate delivery would
   reach MAX_STALLED_COMPACTIONS and abort the session. Re-deliveries are now
   recognized by the absence of any message activity since the previous
   compaction (a genuine new compaction is always preceded by messages, since the
   context has to grow again to trigger one). New messageSeenSinceCompaction
   flag, persisted and restored fail-open.

2. A completion/blocked claim on the retained compaction source is honored.
   The terminal checks ([goal:complete] / [goal:blocked]) are split off from
   the progress/checkpoint boundary. The compaction source still gates
   checkpointing and stall heuristics, but no longer suppresses a terminal claim
   on the retained turn — which had survived the compaction unprocessed, so
   swallowing it discarded a real result and spent another continuation.

3. Remove the inert tool-part detection branch. messageHasToolCall against
   the message.updated envelope is always false (no parts on that event), so it
   was dead. The breaker resets on assistant output-token progress, which a
   tool-using turn also produces. messageHasToolCall is unchanged and still
   correct at its other call site, which reads API-fetched messages that do carry
   parts.

Tests

• Rewrote the productive-turn test onto the canonical properties.info shape with
  realistic output tokens (the original used an envelope OpenCode does not emit and
  output: 0, which rested the assertion entirely on the now-removed branch).
• Added: identity-less re-delivery does not trip the breaker; a completion claim
  on the retained compaction source is honored; identity-less compactions
  separated by message activity still trip the breaker; tool parts on a
  message.updated event are not a productive-turn signal. The first two fail
  against this branch's base with the source fix reverted.

Verification

npm test (394/394), npm run type:check, npm run test:mutation (67/67),
npm run smoke, all green; also green on Node 18.

Not included (deliberately)

• Whether to raise MAX_STALLED_COMPACTIONS or drop the session.abort on trip
  is left as a separate discussion; the false-positive paths that made the abort
  dangerous are closed here.
• The idle-event dedup reads event?.id, but EventSessionStatus /
  EventSessionIdle carry no id either, so seenIdleEventIDs never dedups in
  production. Same class of issue as (1), predates fix: guard goal continuation across compaction epochs #61 — worth its own issue.

Closes #61.

🤖 Generated with Claude Code

[harryzhou2000]

@willytop8 Thanks for your help!