feat(retrieval): the ledger records what was OPENED, not merely what was shown (#4100)

Milestone 386 made a repeat REFERENCED rather than withheld, and the line it
chose says "You saw it earlier this session". Nothing ever checked that. The
arms emit a TEASER — title, trigger, get_rule(N) — so a session can be shown a
rule twenty times and never read a word of it, and a compaction summarises the
teaser away leaving nothing behind. The server was asserting something about
the reader's context it had no way to know.

Three states now, where there were two:

  never surfaced   "it is not in this session's loaded set"
  named, unopened  "Mentioned earlier this session but not opened — read it…"
  opened           "You opened it earlier this session; pull it… again"

The middle one is the honest one and the one that was missing. It keeps the
full invitation, because a session that skipped a teaser is in nearly the
position of one never shown it.

HOW "OPENED" BECOMES OBSERVABLE. A new PostToolUse hook watches the get_rule
call itself and appends to `<sid>.opened.ids`. PostToolUse does fire for MCP
tools — the event's own output schema carries `updatedMCPToolOutput`, which
would be meaningless otherwise — and the matcher is `mcp__.*__get_rule` so the
server segment, which varies by install, is not pinned.

This is NOT the self-report 386 rejected. That objection was to ASKING a model
whether it holds a rule, which is unverifiable. A tool call is an event the
harness reports whether anyone asks. Recording what a session DID and believing
what it SAYS about itself are different kinds of evidence.

Both ledgers clear together on compact/clear. Keeping `.opened.ids` across a
compaction would have the arms telling a freshly-summarised session "you opened
it earlier" about a rule now nowhere in its context — a more confident version
of the bug being removed. Same reader (scribe_rules_live) for both, so ageing,
last-entry-wins and the bare-id format are defined once.

Also closes two smoke-coverage holes the checker was reporting as SKIP: the new
recorder, and scribe_precompact_preserve.sh from #3680. The latter needed
STATIC_FLOOR to become a set — PreCompact's contract is inverted, its stdout
BECOMES the summarizer's instructions, so silence is its failure mode and a
generic read of it looks like a leak.

Step 2 of milestone 416, and a hard prerequisite for step 4: while suppression
keys on shown, widening k marks records "seen" faster than they are read, and
the ledger would degrade in proportion to the improvement.

Plugin minted 2026.09.16.1232 -> 2026.09.16.2102.

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

@@ -105,6 +105,13 @@ case "$source" in
       # Best-effort, like every other filesystem touch in these hooks: a ledger
       # that cannot be removed costs a repeated exclusion, never a session.
       rm -f "${TMPDIR:-/tmp}/scribe-priorart/${safe_sid}.rules.ids" 2>/dev/null || true
+      # BOTH ledgers, for one reason (#4100). `.opened.ids` records what the
+      # session read; a compaction is exactly the event that takes it away
+      # again. Clearing the naming ledger while keeping this one would leave
+      # the surfacing arms telling a freshly-summarised session "you opened
+      # it earlier" about a rule that is no longer anywhere in its context —
+      # a more confident version of the claim this milestone removed.
+      rm -f "${TMPDIR:-/tmp}/scribe-priorart/${safe_sid}.opened.ids" 2>/dev/null || true
     fi
     ;;
 esac