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

plugins/core/skills/agentsmd-author/SKILL.md

@@ -0,0 +1,56 @@
+---
+name: agentsmd-author
+description: >
+  Use when the user wants to create or update a repo's AGENTS.md file
+  ("write an AGENTS.md for this repo", "add setup/test instructions for
+  agents", "update AGENTS.md", "give this package its own AGENTS.md") — even
+  if they don't name the file explicitly, e.g. "document this for AI coding
+  tools" or "make sure agents know how to run tests here". Writes/updates
+  AGENTS.md by exploring the target repo for real build, test, lint, and
+  style conventions — never invents commands. Supports nested monorepo
+  placement (a subdirectory can get its own AGENTS.md following
+  nearest-file-wins precedence). Closes every run by invoking agentsmd-audit
+  inline, and calls provider-adapter-author when an existing provider file
+  (CLAUDE.md, etc.) now duplicates what AGENTS.md owns. Do not use to review
+  an existing AGENTS.md without changing it — use agentsmd-audit instead. Do
+  not use to convert CLAUDE.md/.cursor/rules into a thin adapter — use
+  provider-adapter-author instead.
+allowed-tools: Bash Read Write Edit
+metadata:
+  category: docs
+  source_keys:
+    - agents-md-official
+    - context7-websites-agents-md
+    - context7-agentsmd-agents-md
+  version: "0.1.1"
+---
+
+## Gotchas
+
+- Never invent a command. Every line under a setup/test/build section must come from something you actually found in the repo (`package.json` scripts, a `Makefile` target, a CI workflow step, a README). If you can't verify a command, don't include it.
+- AGENTS.md has no required schema — don't force every common-sections-checklist heading into every repo. Include only sections that reflect something real about this repo; a thin, accurate file beats a padded, generic one.
+- Nested placement is for genuinely different conventions, not convenience. Only create a subdirectory AGENTS.md when that subtree has its own build tool, stack, or conventions distinct from the root — otherwise you're duplicating content the root already covers, which the nearest-file-wins rule doesn't merge back together.
+- This skill never touches CLAUDE.md, `.cursor/rules/*.mdc`, `copilot-instructions.md`, or similar provider files directly — that's `provider-adapter-author`'s job. Detect and hand off; don't reconcile it yourself.
+- This skill never audits on its own judgment — the closing `agentsmd-audit` invocation is mandatory, not optional, even when the change looks trivial.
+
+## Step 1 — Explore the target repo
+
+Before writing anything, gather real facts: package manager and scripts (`package.json`, `pyproject.toml`, `Cargo.toml`, etc.), a `Makefile` or task runner, CI config (`.github/workflows/`, etc.) for the commands it actually runs, linter/formatter config files, and any existing docs (`README.md`, existing `AGENTS.md`) describing conventions. Note whether any subdirectory looks like its own package with a different stack.
+
+## Step 2 — Decide placement
+
+- No `AGENTS.md` at the repo root yet → create one there first, covering whole-repo conventions.
+- A subdirectory has materially different build/test tooling or conventions than the root → create or update a nested `AGENTS.md` there, scoped to what's different. Don't repeat root-level content — the nearest-file-wins rule means the nested file is read alone, not merged with the root.
+- Otherwise → update the existing file(s) in place.
+
+## Step 3 — Write or update
+
+Use only sections that reflect something real about the repo — never fill in every common-sections-checklist heading just because it exists. Read `references/content-guide.md` for section-by-section guidance, a worked example, and what separates useful content from generic padding, before writing.
+
+## Step 4 — Check for an existing provider file
+
+Look for `CLAUDE.md`, `.cursor/rules/*.mdc`, `.github/copilot-instructions.md`, or similar in the target repo. If one exists and now duplicates content the AGENTS.md you just wrote/updated already owns, invoke the `provider-adapter-author` skill on it to reconcile — don't rewrite it yourself.
+
+## Step 5 — Audit and report
+
+Invoke the `agentsmd-audit` skill directly on the AGENTS.md file(s) you just wrote or updated. Resolve any FAIL findings before considering the work done — re-invoke this skill's own writing steps to fix them, then re-run the audit, same as any other close-the-loop check. Report what was created/changed, whether a provider file was reconciled, and the audit's final result.