docs(6209): twelve more test files say which behaviour they pin

shape ledger, MCP projects/search/rulebooks tools, backup, chunking,
plugin context, derived mirror, rule band, shape form gate, plugin
version mint and note usage tests drop issue, milestone and rule numbers,
dates and one-install figures from docstrings, comments and messages.
Three shape-form-gate test names that told the discovery story now name
the property: a registry module's helpers are not service functions, the
divergence gate silences only a family contradiction, and callable helpers
beside a callable canon still prompt. The rulebook tool-count comment that
logged each milestone's additions is replaced by the reason it is a count.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/test_rule_hint_band.py

@@ -1,22 +1,20 @@
-"""An act surfaces a SET of rules, and rank decides how loudly (#3851).
+"""An act surfaces a SET of rules, and rank decides how loudly.
 
 WHY THIS EXISTS
 
-`RULEHINT_LIMIT` was 1. That was right while retrieval merely SUPPLEMENTED a
-33-rule resident set — one salient rule beside everything already loaded. It
-stops being right the moment milestone 394 removes residency, because then
-this arm is the whole delivery, and `git push origin dev` is governed by
-rules 1, 2, 9 and 140 at once, each of which alone permits the mistake the
-others catch.
+This arm is the whole delivery of rules to an act, and one command can be
+governed by several rules at once — a `git push` by where to push, what not
+to push, who polls CI and what to wait for — each of which alone permits the
+mistake the others catch. One slot cannot serve that.
 
 Two instruments, and they answer different questions:
 
   - `_rule_band` decides HOW MANY. A fixed k fills its slots whether or not
     anything deserves them; a band keeps only what scored close to the top,
     so one clearly-relevant rule still shows one.
-  - `compact` decides HOW LOUD. Measured at #3851: a full line is ~143 tokens
-    once the trigger is rendered, so five of them cost ~646 before every Bash
-    call. Top-full-plus-references costs ~198.
+  - `compact` decides HOW LOUD. A full line is ~143 tokens once the trigger
+    is rendered, so five of them cost ~646 before every Bash call.
+    Top-full-plus-references costs ~198.
 
 WHAT THIS PINS
 
@@ -96,7 +94,7 @@ def test_a_flat_cluster_surfaces_together():
 
 
 def test_the_band_is_computed_from_scores_not_from_the_ledger():
-    """A seen top hit still anchors the band (#3750 x #3851).
+    """A seen top hit still anchors the band.
 
     `_rule_band` never learns what the session has seen — dedup happens after
     it, in the arms. Pinned here because the tempting "reorder so a fresh rule
@@ -124,7 +122,7 @@ def test_the_leading_line_carries_the_trigger(seen):
 def test_a_later_line_cites_its_rule_without_quoting_the_trigger(seen):
     """Rank > 0 is a reference: identity and pointer, no trigger.
 
-    The trigger is the expensive half — 300-400 characters after #3855 — and
+    The trigger is the expensive half — often 300-400 characters — and
     the leading line has already demonstrated the shape. Both assertions
     matter: dropping the trigger is the saving, and keeping `get_rule(id)` is
     what makes the saving safe, because a cited rule the reader cannot pull is
@@ -140,12 +138,11 @@ def test_a_later_line_cites_its_rule_without_quoting_the_trigger(seen):
 
 
 def test_shortening_a_line_does_not_decide_what_it_says_about_holding():
-    """A reference still tells a repeat from a first surfacing (#3750 x #3851).
+    """A reference still tells a repeat from a first surfacing.
 
-    This is the regression the first cut of #3851 actually shipped: the
-    compact branch dropped the tail along with the trigger, so a rule the
-    session had already been told read exactly like one it had not. #3750's
-    whole argument is that the two are different claims — a repeat is rendered
+    A compact branch that drops the tail along with the trigger makes a rule
+    the session had already been told read exactly like one it had not. The
+    two are different claims — a repeat is rendered
     precisely because the session may no longer HOLD what it was told — and
     the tail is the entire difference a reader can act on.
 
@@ -153,10 +150,8 @@ def test_shortening_a_line_does_not_decide_what_it_says_about_holding():
     a fact about its rank; what the session holds is a fact about the ledger;
     and neither may be allowed to answer the other's question.
 
-    THREE STATES SINCE #4100, so this checks three. The axis grew and the test
-    grew with it — pinning only two would leave the compact branch free to
-    collapse the new middle state into either neighbour, which is the same
-    regression this was written for with one more place to hide.
+    THREE STATES, so this checks three. Pinning only two would leave the
+    compact branch free to collapse the middle state into either neighbour.
     """
     rule = fake_rule(id=4, title="dev is home", when_to_apply=_TRIGGER)
     fresh = _rule_hint_line(rule, where="here", seen=False, compact=True)
@@ -182,8 +177,8 @@ def test_a_compact_line_is_materially_shorter_than_a_full_one():
     if a later edit puts the trigger back into the compact branch.
 
     Measured against a REALISTIC trigger, because that is where the saving
-    lives: the rules this arm carries run 300-400 characters of trigger after
-    #3855, and a toy one-line trigger would make this pass on a compact branch
+    lives: the rules this arm carries run 300-400 characters of trigger, and
+    a toy one-line trigger would make this pass on a compact branch
     that had stopped saving anything.
     """
     long_trigger = (
@@ -200,7 +195,7 @@ def test_a_compact_line_is_materially_shorter_than_a_full_one():
 
 
 def test_a_preference_keeps_its_noun_when_compact():
-    """Force survives the shortening (#3849).
+    """Force survives the shortening.
 
     `kind` and `compact` are independent axes. A preference rendered as a
     reference must still not read as a rule — the noun is the whole of the