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

tests/test-vale-wrap.sh

@@ -460,11 +460,11 @@ fi
 #   - tests/*.sh               — the runners and every regression test
 #   - plugins/*/.apm/**/*.sh   — the scripts plugins ship to users
 #   - providers/**/*.sh        — the scripts install.sh deploys to user machines
-# plugins/*/skills/** is deliberately NOT scanned: it is the generated mirror of
-# .apm/, so scanning both double-reports every finding, and mirror-vs-source
-# drift is already sync-plugin-content.sh --check's job. Scanning .apm/ is what
-# closed the gap where skill-audit's vale-wrap.sh was covered but agent-audit's
-# byte-identical copy of it was not.
+# .apm/ is the only plugin content scanned, and it is the only plugin content
+# there is: the generated flat plugins/*/skills/** mirror was deleted with
+# native plugin-install support, so there is no second copy to double-report.
+# Scanning .apm/ is what closed the gap where skill-audit's vale-wrap.sh was
+# covered but agent-audit's byte-identical copy of it was not.
 # providers/**/*.sh was excluded until issue #96: statusline-command.sh seeded
 # `parts=()` empty and expanded it unguarded, a real latent hazard rather than a
 # false positive, in a file this case did not own. That expansion is now guarded,