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

.gitignore

@@ -47,8 +47,8 @@ build/
 
 # `apm pack`'s manifest for the *root* package. Emitted beside the marketplace
 # manifest by a bare `apm pack`, and never tracked on any branch — the repo's
-# own paths hide it, since sync-plugin-content.sh redirects `apm pack -o` to a
-# scratch tree and the apm-pack-check-clean pre-push hook runs --dry-run. Scoped
+# own paths hide it, since the apm-pack-check-clean pre-push hook, the only
+# thing that runs pack here, passes --dry-run. Scoped
 # to the file, not the directory: the sibling .claude-plugin/marketplace.json is
 # compiled output that IS committed and must stay tracked.
 /.claude-plugin/plugin.json