fix(kyberforge): keep .github/plugin/marketplace.json synced

apm.yml only declares claude and codex marketplace output profiles --
codex writes a differently-shaped file to .agents/plugins/marketplace.json,
so nothing regenerates the legacy Copilot CLI path at
.github/plugin/marketplace.json. It was hand-synced once during the #90
conversion and had drifted (missing the category field added when codex
output was enabled) with no gate to catch it.

scripts/sync-marketplace-mirror.sh keeps it byte-identical to the compiled
.claude-plugin/marketplace.json, wired as a pre-push check so it can't go
stale silently again.

Refs: #90

CONTEXT.md

@@ -30,7 +30,7 @@ Reusable slash commands for AI coding tools, defined as `SKILL.md` files followi
 The deployable unit in the plugin marketplace. A plugin bundles one or more skills, agents, hooks, prompts, MCP servers, and optionally a `bin/` directory into a single installable directory. In this repo, plugins live under `plugins/<name>/`, each with its own `apm.yml` + `.apm/{skills,agents,hooks,...}` — this is the authoring source of truth for the plugin's content (ADR-0015). Two categories of tracked output are compiled from that source, never hand-edited: `.claude-plugin/plugin.json` (Claude Code) and `.github/plugin/plugin.json` (Copilot CLI) via `apm pack`/`apm compile`; and, alongside them, a flat `agents/`, `skills/`, `commands/`, `instructions/`, `extensions/` directory mirror plus a merged `hooks.json` at the plugin root, generated by `scripts/sync-plugin-content.sh` — Claude Code's and Copilot's installers convention-scan only these flat root-level paths and have no awareness of `.apm/` nesting at all, so this mirror is what actually makes `.apm/` content discoverable at install time (ADR-0017). Plugins are copied to a cache on install — they cannot reference files outside their own directory. Install a plugin with `claude plugin install <name>@<marketplace>`.
 
 ### Plugin marketplace
-A Git repository with a `marketplace.json` manifest listing installable plugins. No backend, registry, or SaaS required — the Git repo is the marketplace. This repo is the `holocron` marketplace. The manifest at `.claude-plugin/marketplace.json` (read by both Claude Code and Copilot CLI, mirrored to `.github/plugin/marketplace.json`) is **compiled output** of `apm pack`, generated from the root `apm.yml`'s `marketplace:` block (owner, build/output config, versioning strategy, and the `packages:` list of installable plugins) — it is not hand-edited. See ADR-0015. Each listed package's `source:` still points at that plugin's own `plugins/<name>/` root, not at an `apm pack` build artifact — which is why that root also carries the flat `agents/`/`skills/`/`commands/`/`hooks.json` content mirror described under "Plugin" (ADR-0017): without it, an install from this marketplace finds a valid manifest but no discoverable content.
+A Git repository with a `marketplace.json` manifest listing installable plugins. No backend, registry, or SaaS required — the Git repo is the marketplace. This repo is the `holocron` marketplace. The manifest at `.claude-plugin/marketplace.json` (read by both Claude Code and Copilot CLI) is **compiled output** of `apm pack`, generated from the root `apm.yml`'s `marketplace:` block (owner, build/output config, versioning strategy, and the `packages:` list of installable plugins) — it is not hand-edited. See ADR-0015. `.github/plugin/marketplace.json` is Copilot CLI's legacy manifest path; apm has no output profile for it (only `claude` and `codex`, and `codex`'s is a differently-shaped file at `.agents/plugins/marketplace.json`), so `scripts/sync-marketplace-mirror.sh` keeps it byte-identical to `.claude-plugin/marketplace.json`, checked at pre-push. Each listed package's `source:` still points at that plugin's own `plugins/<name>/` root, not at an `apm pack` build artifact — which is why that root also carries the flat `agents/`/`skills/`/`commands/`/`hooks.json` content mirror described under "Plugin" (ADR-0017): without it, an install from this marketplace finds a valid manifest but no discoverable content.
 
 ### HITL (human-in-the-loop)
 Agent pauses before a consequential action; human approves before execution. Required for irreversible or high-stakes actions (architecture changes, production deployments, security configuration). The agent drafts the change plan and waits — it does not proceed autonomously. Contrast with HOTL.