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/bin/skills/research/references/file-format.md

@@ -0,0 +1,39 @@
+# Reference file format
+
+Every topic file follows this structure.
+
+## Frontmatter
+
+```yaml
+---
+topic: <topic-slug>          # matches the filename without .md (e.g. "api-reference")
+source_keys:                 # kebab-case slugs of sources that contributed; must match sources.md entries
+  - <slug>
+  - <slug>
+---
+```
+
+## Body
+
+Plain prose organized into markdown sections (`##`, `###`). Extract the content most relevant to skill authoring or implementation — not a verbatim copy of the source. Focus on:
+- Decisions that affect how to call the API or tool
+- Options, flags, or parameters with non-obvious behavior
+- Constraints, rate limits, or gotchas
+- Canonical patterns the skill should follow
+
+No inline URLs in the body — all source traceability lives in `sources.md` via `source_keys`.
+
+## sources.md format
+
+```markdown
+# Sources
+
+## <slug>
+
+- **URL:** <full URL>
+- **Description:** <one-line summary of what this source covers>
+- **Contributing files:** <comma-separated list of topic files this source contributed to>
+- **Status:** `extracted` | `no content extracted`
+```
+
+Use one `##` section per source. Slugs are kebab-case derived from the domain or page title (e.g. `stripe-api-docs`, `openai-python-sdk-readme`). For Context7 sources, use the slug `context7-<library-slug>` (e.g. `context7-vercel-next-js`) and set **URL** to `context7:<library-id>` (e.g. `context7:/vercel/next.js`).