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-bats.sh

@@ -28,6 +28,13 @@ fi
 # relative paths (`$BATS_TEST_DIRNAME/../../../../../../`) resolve to the
 # dependency's root, not this repo's, so they fail on a missing bats-support
 # helper. They are the same tests already discovered under plugins/.
+#
+# .claude/skills/ is excluded for the same reason, one install step later. apm
+# deploys skills there straight from each plugin's `.apm/` tree, `tests/` dirs
+# and all, so every `<skill>/tests/*.bats` file under plugins/ also lands at
+# .claude/skills/<name>/tests/ and would be discovered and run a second time.
+# (It used to be invisible here: the generated flat mirror apm installed from
+# stripped `tests/` out. The mirror is gone, so the exclusion is explicit.)
 TEST_FILES=()
 while IFS= read -r f; do
     TEST_FILES+=("$f")
@@ -37,6 +44,7 @@ done < <(
         -not -path "*/test_helper/*" \
         -not -path "*/.claude/worktrees/*" \
         -not -path "*/apm_modules/*" \
+        -not -path "*/.claude/skills/*" \
         | sort
 )
 
@@ -75,7 +83,7 @@ if [[ -n "$GIT_TOPLEVEL" && "$GIT_TOPLEVEL" == "$REPO_ROOT" ]]; then
         [[ -n "$f" ]] && EXPECTED_FILES+=("$REPO_ROOT/$f")
     done < <(
         git -C "$REPO_ROOT" ls-files -- '*.bats' \
-            | grep -Ev '(^|/)tests/bats/|(^|/)test_helper/|(^|/)\.claude/worktrees/|(^|/)apm_modules/' \
+            | grep -Ev '(^|/)tests/bats/|(^|/)test_helper/|(^|/)\.claude/worktrees/|(^|/)apm_modules/|(^|/)\.claude/skills/' \
             | sort || true
     )
 else
@@ -120,9 +128,8 @@ fi
 # flushed in stable sorted order once every job has finished.
 #
 # Dispatch and throttling is scripts/lib/batch-run.sh's batch_run -- shared
-# with scripts/sync-plugin-content.sh and tests/run-tests.sh so a batching bug
-# fix only needs to land once; see that file for why this is batched rather
-# than a rolling `wait -n` pool.
+# with tests/run-tests.sh so a batching bug fix only needs to land once; see
+# that file for why this is batched rather than a rolling `wait -n` pool.
 SCRATCH_ROOT="$(mktemp -d)"
 trap 'rm -rf "$SCRATCH_ROOT"' EXIT
 # Repo-root-relative -- see tests/run-tests.sh for why `../scripts/...` does not