fix(kyberforge): bridge apm content to Claude Code's flat plugin discovery

Claude Code's (and Copilot's) native plugin installer has zero awareness of
.apm/ nesting -- it convention-scans only flat skills/, agents/, commands/,
hooks.json at each plugin's root. Confirmed via strings on the installed
claude binary and live installs of git@holocron/gitea@holocron/kyberforge@
holocron, all reporting Skills(0) Agents(0) Hooks(0) post ADR-0015's apm
conversion. Root cause (apm_cli/core/plugin_manifest.py): apm's plugin.json
compiler deliberately strips skills/agents/commands keys, assuming the host
already auto-discovers those convention directories -- it has no model of
.apm/ being host-visible at all. Separately, apm's own bundle exporter
(apm_cli/bundle/plugin_exporter.py, behind `apm pack --format plugin`)
implements the correct .apm/ -> flat mapping, but only ever targeted
build/<name>-<version>/, a path nothing in marketplace.json's source: points
at.

scripts/sync-plugin-content.sh wraps that bundle exporter and copies its
agents/, skills/, commands/, instructions/, extensions/, and merged
hooks.json back into each plugin's own root as a second tracked
compiled-output category -- same governance status as
.claude-plugin/plugin.json: generated from .apm/, never hand-edited. tests/
subdirectories are excluded from the mirror (dev fixtures, not host-visible
runtime content; several hardcode a relative repo-root walk-up sized for the
.apm/-nested depth, which breaks when duplicated one level shallower).
Applied for real across all 6 plugins and verified two ways: `claude plugin
validate --strict` passes on every real plugin directory, and a live
`claude --plugin-dir <path> -p "list skills/agents"` behavioral test
confirms content is now actually discovered.

Also, from the same issue #90 review round:
- scripts/check-manifests.sh pointed at each plugin's root-level plugin.json
  (checking skills/hooks/mcpServers/agents pointer fields) -- that file was a
  stale near-duplicate of .claude-plugin/plugin.json nothing else read or
  wrote, now deleted across all 6 plugins. check-manifests.sh is rewritten to
  validate .claude-plugin/plugin.json instead, and drops the pointer-field
  checks entirely (nothing to check -- those fields are correctly absent by
  design). Content-presence drift is now check-plugin-content-sync's job, a
  new pre-push hook wired in .pre-commit-config.yaml.

docs/adr/0017 records the root cause and decision in full, including two
rejected alternatives (patching plugin.json's path fields directly -- apm's
compiler strips them on every run; pointing marketplace.json at apm pack's
build/ output -- a version-suffixed non-source directory nothing can install
from without an extra build step). ADR-0015 and CONTEXT.md are updated to
point at it.

Refs: #90

CONTEXT.md

@@ -27,10 +27,10 @@ A separate product (separate repo) for browsing, editing, and configuring AI dev
 Reusable slash commands for AI coding tools, defined as `SKILL.md` files following the [Agent Skills open standard](https://agentskills.io). Deployed via plugin — `plugins/<plugin-name>/.apm/skills/<skill-name>/SKILL.md`, available after the plugin is installed (`claude plugin install <name>@<marketplace>`). Skills are self-contained — they cannot reference files outside the plugin directory after install-time caching.
 
 ### Plugin
-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). `.claude-plugin/plugin.json` (Claude Code) and `.github/plugin/plugin.json` (Copilot CLI) are **compiled output** of `apm pack`/`apm compile`, generated from `apm.yml` + `.apm/` — they are not hand-edited. 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>`.
+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.
+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.
 
 ### 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.