feat(plugin): a PreCompact hook tells the summarizer what must survive (#3680)

Spike #3680 asked whether a PreCompact hook can reach the model. Read out of
the installed Claude Code build (2.1.273), the answer is yes — through a
different channel than note #3679 assumed:

  * `hookSpecificOutput.additionalContext` is NEVER read on PreCompact. The
    hook-output schema has no PreCompact variant; the field is honoured for
    SessionStart, SubagentStart and Stop, and silently dropped here.

  * A PreCompact hook's STDOUT becomes `newCustomInstructions`, merged with
    the operator's own `/compact` instructions and passed into the prompt that
    writes the summary. Manual, auto and partial compaction all do this.

So the hook does not interrupt the compaction — it steers the summary, which
is what the next turn reads. Blocking is the thing not to do: a PreCompact
block SKIPS compaction, tells the model nothing, and leaves the session
running on uncompacted with no summary at all.

scribe_precompact_preserve.sh names what the summary is the only copy of:
Scribe record ids WITH titles, the in-progress task and its milestone, work
done but not yet recorded, governing rules, and unfinished operator asks.

No network and no config — what must survive is already in the conversation
being summarized; the hook only says which parts are load-bearing.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01821k5B3Ysecp9fNYs92Kuy

plugin/.claude-plugin/plugin.json

@@ -1,7 +1,7 @@
 {
   "name": "scribe",
   "description": "Scribe for Claude Code: connects the scribe MCP server, adds the hooks that deliver live project state and relevant records at the right moment, ships the shared client-neutral Scribe skills (using-scribe, writing-plans, reporting-back, systematic-debugging, verification, brainstorming, reusing-code, shape-accounting), and syncs your saved Scribe Processes as skills (/scribe:sync).",
-  "version": "2026.09.15.1921",
+  "version": "2026.09.16.1211",
   "author": {
     "name": "Bryan Van Deusen"
   },