chore: drop the flat content mirror and native install support (ADR-0024)

apm becomes the only supported install path. The flat mirror at each plugin
root existed solely so Claude Code's native `claude plugin install` could
convention-scan plugin content (ADR-0017). With no native consumers, it cost
~20,000 tracked lines plus ~2,100 lines of sync tooling and ~88s of every
push to guard content apm never reads — and its only automated gate,
`claude plugin validate --strict`, passes on a plugin with zero content, so
it could not detect the defect ADR-0017 was created to fix.

Removes the mirror (213 files), the six per-plugin manifest pairs,
sync-plugin-content.sh, its 1,289-line test, the orphaned
marketplace-plugins.sh, and the check-plugin-content-sync and
validate-plugins pre-push hooks. The root `marketplace:` block and
.claude-plugin/ catalogue stay: apm's own marketplace consumers read that
same file, so `<name>@holocron` short names keep working.

tests/run-bats.sh now excludes .claude/skills/. apm installs from .apm/,
which carries the tests/ dirs the mirror stripped, so deployed .bats files
would otherwise be discovered and double-run.

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

docs/adr/0017-plugin-content-mirror-bridges-apm-to-host-discovery.md

@@ -1,5 +1,17 @@
 # Plugin roots gain a compiled flat-directory mirror of `.apm/` content so Claude Code can discover it
 
+**Superseded by:** ADR-0024 (apm is the only supported install path; the flat content mirror is
+deleted). The mirror this ADR created — `plugins/<name>/{skills,agents,hooks}/` — has been deleted,
+along with `scripts/sync-plugin-content.sh`, its test suite, and the `check-plugin-content-sync`
+pre-push gate. Native `claude plugin install` is no longer a supported path, so the host discovery
+contract this ADR bridged is no longer one this repo satisfies. The diagnosis below is still
+accurate about how Claude Code's installer works; what changed is that nothing consumes it. The
+`mcpServers`, `hooks`-pointer and `hooks/hooks.json` amendments below are moot with the artifacts
+they governed; the symlink amendment's underlying gap is not — see ADR-0024's consequences. This
+ADR's content is kept below as the historical record; it is no longer the current model.
+
+---
+
 This ADR is a follow-on correction to ADR-0015 (Microsoft APM replaces hand-authored
 plugin/marketplace authoring), discovered during issue #90's post-execution review. It does not
 restate ADR-0015's rationale for adopting `.apm/` as the authoring source of truth — see that ADR