docs(self-surfacing): derive groups are drift not audit material — shape-accounting + reusing-code skills, static floor, plugin README (after-write hook), api-reference rows (#2902, milestone 299 step 4)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

plugin/skills/reusing-code/SKILL.md

@@ -44,6 +44,13 @@ through recall/auto-inject; this skill is the active reflex around that.
   it before you go any further. Either it's the helper you were about to
   duplicate — reuse it and drop yours — or it isn't, and the record needs the new
   location adding. Both are cheaper now than after the duplicate settles in.
+- **A `Shape ledger at …` line is the ledger speaking, not the record.** It
+  names a duplicate family ("identical body in N other files, no canon") or a
+  canon elsewhere for a name you just wrote — for edits made through Bash
+  (sed, heredocs, scripts) as much as through Write/Edit. Derive the family or
+  reuse the canon *now*; a family that is convention rather than copies is
+  dismissed with `classify_shapes(..., status="exempt",
+  reason_code="convention-plumbing")`, never ignored.
 - **A `[records this file]` hint is a duty, not a menu.** When the hint says a
   snippet records the very file you're editing, the record's freshness is now
   YOUR edit's responsibility: if the edit changes the recorded shape,

plugin/skills/shape-accounting/SKILL.md

@@ -85,6 +85,34 @@ the dominant form, `create_snippet` it, migrate the outliers, then classify
 the rest as instances. Canon is determined from the code; consistency comes
 from the derivation, not from asking permission.
 
+## Derive groups are drift, not audit material
+
+The catalogue exists so the codebase is DRY **from inception**, not as DRY as
+the last sweep left it. Three surfaces say so without anyone running an audit
+(milestone 299):
+
+- **At the write** — the prior-art hint (the Write/Edit hook, and since
+  0.1.39 the after-write hook on Bash, so sed/heredoc/script edits count too)
+  carries a `Shape ledger at <path>` line when a name just written is a known
+  **duplicate family** ("identical body in N other files, no canon") or a
+  **canon elsewhere** ("snippet #N at <path> — reuse, don't redefine"). Act
+  on it *then*: pull the canon and build from it, or derive the family now —
+  `create_snippet` the dominant form, repoint the copies, `classify_shapes`
+  them `instance`. A family is named once per session.
+- **On arrival** — the coverage line's `standing:` block (shown even when
+  nothing is unclassified) and `derive_new` ("+N new copies since last
+  refresh: .x in <path>") name what drifted since the previous refresh. That
+  is the todo of the moment, sized to the last batch — not a backlog.
+- **A family that is convention, not copies** — component-local `load` /
+  `toggle` / `save` that happen to share a name — is dismissed, not
+  consolidated: `classify_shapes(..., status="exempt",
+  reason_code="convention-plumbing", reason=…)` (or `classify_shapes_by_rule`
+  for a whole family) removes it from the queue. Dismissal is a judgment and
+  it is recorded; silence is not.
+
+After the one-time pay-down the derive queue reads empty; anything in it
+afterwards is drift of the moment, and the hint already said so at the write.
+
 ## The divergence readout — button B where button A is canon
 
 Three questions the ledger answers mechanically (#2793):