docs: drift ratchet in CONTRIBUTING's static tests; /learn prune example asks per entry

Co-authored-by: capy-ai[bot] <230910855+capy-ai[bot]@users.noreply.github.com>

CONTRIBUTING.md

@@ -429,6 +429,7 @@ eval files, and misses the strict classifier. No API keys needed.
 - **Tier-alignment invariant** (`test/e2e-tier-alignment.test.ts`) — For every self-gated `test/skill-e2e-*.test.ts` named in a touchfiles dep list, the file's `EVALS_TIER` self-gate must match its declared tier in `E2E_TIERS`. Kills the "inert demotion" class where a test is re-tiered in `touchfiles.ts` but the file still gates on the old tier and keeps running in the wrong lane. Unmapped or mixed-tier files are reported, never silently skipped.
 - **Catalog budget** (`test/catalog-budget.test.ts`) — Caps the aggregate discovery surface: the sum of every skill's frontmatter `name` + `description` (what every host loads at discovery, every session) must stay under 1,393 token-equivalents, with a 260-byte per-skill cap. Counting goes through the shared census in `test/helpers/skill-census.ts` (physical files vs authored skills vs registry entries — three deliberately different counts). Adding a skill? The failure message carries the re-measure + ratchet protocol.
 - **Context-budget ratchet** (`test/context-budget-ratchet.test.ts`) — CI ceilings on the two token ledgers the catalog budget doesn't cover: the always-on full-frontmatter aggregate and each skill's per-invocation eager tokens (SKILL.md + forced-read references), graded against `test/fixtures/context-budget.json` via `lib/context-bill.ts`. New skills fail until they have a ceiling; ceilings for removed skills must be pruned. Legitimate growth or a landed reduction: re-run `bun test/helpers/capture-context-budget.ts` and commit the refreshed fixture in the same commit, so the change is a visible decision in the diff.
+- **Skill-to-bin drift ratchet** (`test/skill-bin-drift.test.ts`) — `scripts/skill-bin-drift.ts` statically reads every gstack bin call in skill templates, carved sections, references and the Claude-host renders (it never executes a bin) and fails with `DRIFT_UNKNOWN_BIN` or `DRIFT_UNKNOWN_FLAG` at file:line when a skill names a bin missing from `bin/` or a flag that bin's parser does not accept. Known misses live in `test/fixtures/skill-bin-drift-allowlist.json`, which only shrinks: a new finding fails, and so does an entry that no longer reproduces. Check one skill with `bun run scripts/skill-bin-drift.ts --skill <name>`; bins with permissive parsers or no usage text are reported, never failed.
 - **Dependency security regressions** (`test/dependency-security.test.ts`) — Run `bun test test/dependency-security.test.ts` to check the resolved `sharp` and `adm-zip` version floors, load Sharp, verify ordinary ZIP extraction, and reject extraction through destination-file and destination-directory symlinks. The symlink cases skip Windows. These checks complement the OSV scan; they do not change its existing exceptions.
 
 ### Tier 2: E2E via `claude -p`

docs/skills.md

@@ -1269,7 +1269,7 @@ Claude: 23 learnings for this project (14 high confidence, 6 medium, 3 low)
 
         3 potentially stale (referenced files deleted):
         - "auth middleware uses JWT" — auth/middleware.ts was deleted
-        [Prune these? Y/N]
+        [Remove, keep, or update each one?]
 ```
 
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