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

AGENTS.md

@@ -9,10 +9,6 @@ This file carries only what applies to **every** session. Setup, prerequisites,
 - `plugins/` — six installable plugin units, each an apm package (`apm.yml` + `.apm/`). Root `apm.yml` declares all six as `dependencies.apm`; `apm install` deploys them into `.claude/skills/` and `.claude/agents/`, both gitignored install output.
 - `providers/claude-code/` — Claude Code adapter, deployed to `~/.claude/` via `scripts/install.sh`.
 
-## Edit `.apm/`, never the flat mirror
-
-`plugins/<name>/.apm/` is the only hand-edited source for plugin content — the generated flat mirror and manifests are discarded on the next sync and reported as drift by `check-plugin-content-sync`. What's exempt (`README.md`, `docs/`, `bin/`, `sources.md`, `.mcp.json`) and why: `docs/spec/architecture.md`.
-
 ## Prefer plugin skills over raw shell
 
 This repo dogfoods its own plugins. Before shelling out, check whether a skill already owns the operation — it usually does: