chore: drop redundant apm-marketplace-check and Copilot marketplace mirror (findings 1, 2)

apm-marketplace-check checked network reachability of remote marketplace
refs, but finding 35 already removed the only remote package entry, so
every marketplace.packages[] source is local and the hook is pure
overlap with apm-pack-check-clean. apm-audit-ci was re-examined and kept
as-is -- its pre-commit-config.yaml comment already carries a dated,
verified justification the audit had missed.

check-marketplace-mirror-sync guarded .github/plugin/marketplace.json
against drift from .claude-plugin/marketplace.json. Verified against
current GitHub Copilot CLI docs: Copilot's marketplace discovery already
falls back through .github/plugin/marketplace.json to
.claude-plugin/marketplace.json, which this repo generates anyway -- the
dedicated mirror bought a discovery-order preference, not a capability.
Deleted the mirror file, its sync script, its test, and the hook.

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

scripts/sync-plugin-content.sh

@@ -196,9 +196,9 @@ if [[ "$ALL" -eq 1 ]]; then
   # duplicate at every call site (see .pre-commit-config.yaml's
   # check-plugin-content-sync).
   #
-  # Hard error rather than a `|| pwd` fallback, on scripts/sync-marketplace-mirror.sh's
-  # reasoning: --all's entire work list hangs off REPO_ROOT, so a REPO_ROOT pointing at
-  # something that is not this repo checks a plugin set that is not this repo's. Run
+  # Hard error rather than a `|| pwd` fallback: --all's entire work list hangs off
+  # REPO_ROOT, so a REPO_ROOT pointing at something that is not this repo checks a
+  # plugin set that is not this repo's. Run
   # from outside a worktree the fallback happens to hit the `--all requires ...` error
   # below instead -- but only by accident, because $PWD had no marketplace.json in it;
   # $PWD holding an unrelated one is the case that would silently "pass".