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

plugins/kyberforge/.apm/skills/agent-author/scripts/README.md

@@ -10,4 +10,4 @@ Usage: new-agent.sh <agent-name> <root>
 
 Resolves scope by walking up from `<root>`: a `type:`-bearing `apm.yml` found at or above `<root>` → plugin/APM scope (single file at `<package-root>/.apm/agents/<name>.agent.md`; an `apm.yml` without `type:` is a marketplace-only manifest and is skipped); `<root>` exactly `~` → user scope (`~/.claude/agents/` + `~/.copilot/agents/`); otherwise project scope (`<root>/.claude/agents/` + `<root>/.github/agents/`). Each file is a no-op if it already exists. See `--help` for full usage.
 
-Tests: `tests/new-agent.bats` (requires `bats-support` and `bats-assert`) — source-only. `scripts/sync-plugin-content.sh` strips `<category>/<name>/tests` from the generated mirror (ADR-0017), so this file exists in a repo checkout of `.apm/skills/agent-author/` and not in an installed plugin.
+Tests: `tests/new-agent.bats` (requires `bats-support` and `bats-assert`).