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/hooks/scribe_session_context.sh

@@ -24,9 +24,15 @@
 # fires after a compaction (SessionStart input `source` == "compact"), when
 # earlier turns have just been summarized and in-flight state is most at risk.
 # On that source we lead with a banner telling the model to reload project +
-# in-flight tasks from Scribe. (PreCompact is the wrong tool here — a host hook
-# can't make the model flush, and can't know the in-flight task ids; the durable
-# path is record-as-you-go + this post-compaction reload.)
+# in-flight tasks from Scribe. This stays the durable path: record-as-you-go
+# plus a post-compaction reload from the instance.
+#
+# The other half arrived with #3680. A host hook still cannot make the model
+# flush before a compaction — but scribe_precompact_preserve.sh steers the
+# SUMMARY, because a PreCompact hook's stdout becomes the compaction's custom
+# instructions. The two are complementary, and neither replaces the other: that
+# hook decides what survives into the summary, this one reloads from the record
+# once the summary lands.
 #
 # IMPORTANT: do NOT pass config via `${user_config.*}` substitution in a
 # shell-form hooks.json command — Claude Code rejects that outright (splicing a