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/run-tests.sh

@@ -156,9 +156,8 @@ done < <(
 # write back into the live repo tree -- verified before adding this), so they
 # run concurrently in fixed-size batches instead of one at a time. Dispatch and
 # throttling is scripts/lib/batch-run.sh's batch_run -- shared with
-# scripts/sync-plugin-content.sh and tests/run-bats.sh so a batching bug fix
-# only needs to land once; see that file for why this is batched rather than a
-# rolling `wait -n` pool.
+# tests/run-bats.sh so a batching bug fix only needs to land once; see that
+# file for why this is batched rather than a rolling `wait -n` pool.
 SCRATCH_ROOT="$(mktemp -d)"
 trap 'rm -rf "$SCRATCH_ROOT"' EXIT
 # The source= path below is repo-root-relative, matching scripts/install.sh:5 --