docs: amend ADR-0021 and close the loose ends ADR-0024 left behind

A six-agent review of today's commits found three documentation defects
that the apm-only change left behind. All three are records that describe
deleted machinery in the present tense; no code or gate is affected.

ADR-0021 was the one ADR that ADR-0024 invalidates without carrying an
amendment note -- 0001, 0006, 0011, 0013, 0014, 0015, 0017, 0018 and 0019
all got one. Its Context section still compiles a plugin description into
four generated files and its Consequences section still names eight, but
718c79a deleted the per-plugin manifest pairs and 0dffff3 deleted the
.github/plugin/marketplace.json mirror. One target survives. The decision
itself is untouched: the note marks the counts historical rather than
rewriting them, since the staleness hazard that motivated the ADR is
exactly what shrinking the blast radius does not fix.

That note also lands the one ADR-0021 has promised since it was written:
its Context section said "see the note below" about the codex profile's
removal and no such note has ever existed in the file.

SIMPLIFICATION-AUDIT.md's section 8 still asked whether Copilot reads the
legacy mirror path. Finding 2c answered that at 11:35 and 0dffff3 acted on
it; two later passes over section 8 (d2480b8, 061bb3d) each checked off a
different question and stepped over this one. Closed with the answer that
already shipped: Copilot's discovery falls through to .claude-plugin/,
so what the deletion cost is discovery-order preference, not consumability.

ADR-0020 cited plugins/bin/skills/zoom-out/SKILL.md:4 as end-to-end
verification evidence. That path is mirror, deleted by ADR-0024. The .apm/
source and the deployed copy both still carry the flag and the pass-through
still holds, so the citation is narrowed to the two live paths rather than
the finding being withdrawn.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YR2CjVumUbEGWcMikcoXBD

SIMPLIFICATION-AUDIT.md

@@ -222,7 +222,8 @@ Findings 9, 10, 11, and 12 are coupled through the provenance validator and the
 
 - [x] ~~**Native Claude Code marketplace install vs apm-only.** The flat mirror, `check-plugin-content-sync`, and ADR-0017 exist only for native `claude plugin install`. If apm install is the only supported path, the mirror and its 2,100 lines of tooling go away. Which install paths must work for consumers?~~
   > **Answered (2026-09-14):** apm-only. See ADR-0024 (`docs/adr/0024-apm-is-the-only-supported-install-path.md`) and commit `718c79a` on `docs/simplification-audit`. Native `claude plugin install` support is dropped; the flat mirror, the twelve per-plugin manifests, `sync-plugin-content.sh`, its test suite, `lib/marketplace-plugins.sh`, and the `check-plugin-content-sync` and `validate-plugins` hooks are all deleted (245 files changed, −22,602 lines). ADR-0017 carries a superseded banner. Kept deliberately: the root `marketplace:` block and the root `.claude-plugin/marketplace.json`, which apm's own consumers read. (`marketplace.json` is the only tracked file under `.claude-plugin/`; the root `plugin.json` beside it is untracked local `apm pack` output, ignored at `.gitignore:59`.) This answer is what voided finding 7's recommendation and closed §3's `check-plugin-content-sync` target.
-- **Copilot CLI legacy path.** Is `.github/plugin/marketplace.json` still read by any Copilot version you target? If not, finding 2c is a pure delete.
+- [x] ~~**Copilot CLI legacy path.** Is `.github/plugin/marketplace.json` still read by any Copilot version you target? If not, finding 2c is a pure delete.~~
+  > **Answered (2026-09-14):** yes, but only as a preferred path, not a required one — so the delete holds. Settled under finding 2c above and executed in commit `0dffff3`; this bullet was left open by oversight when that finding closed. Copilot CLI falls through `marketplace.json`, `.plugin/marketplace.json`, `.github/plugin/marketplace.json`, `.claude-plugin/marketplace.json` in order, and the `.claude-plugin/` file apm already emits satisfies the last step. What was lost is discovery-order preference, not Copilot consumability.
 - **Provenance chain.** Is "which upstream informed this file" a requirement you still want, or was it a governance experiment? Finding 11 hinges on this.
 - **ADR-0012 (three core skills) and the one-script-per-skill install constraint.** The merges in 14, 15, and 24 need the first revisited and are the only way around the second. Are you open to superseding ADR-0012?
 - **Granularity of git/gitea skills.** One `git` skill vs seven trades routing precision for size. Is one broad description acceptable?