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/kyberforge/skills/skill-author/assets/templates/references/README.md

@@ -0,0 +1,31 @@
+# references/
+
+Additional documentation agents load on demand. Files here extend SKILL.md
+without bloating its core context.
+
+## When to add a reference file
+
+Move content here when SKILL.md is approaching 500 lines, or when a topic
+is only relevant in specific circumstances (error handling, edge cases,
+domain-specific sub-procedures).
+
+## How to reference from SKILL.md
+
+Load conditionally — tell the agent exactly when to read each file:
+
+```markdown
+If the API returns a non-200 status, read `references/api-errors.md`.
+```
+
+Avoid generic "see references/ for details" — the agent loads context on
+demand, so give it a precise trigger condition.
+
+## File conventions
+
+- One topic per file — focused files mean less unnecessary context loaded
+- Kebab-case filenames (e.g. `api-errors.md`, `output-formats.md`)
+- Keep files under 200 lines where possible
+
+## If no reference files are needed
+
+Delete this README and the `references/` directory entirely.

plugins/kyberforge/skills/skill-author/assets/templates/references/sources.md

@@ -0,0 +1,14 @@
+# Sources
+
+<!-- Populated at Step 5 of skill authoring, after all skill files are written.
+     For each research source with status `extracted`, record which skill files
+     it contributed to under Contributing files.
+     Delete this file if no research sources were provided as input. -->
+
+## FILL IN: source-slug
+
+- **URL:** FILL IN
+- **Description:** FILL IN
+- **Research doc:** FILL IN: path to the plugin-level research sources file this entry came from (e.g. plugins/myplugin/docs/research/docs/<topic>/sources.md), relative to repo root
+- **Contributing files:** FILL IN: comma-separated list of skill files this source informed (e.g. SKILL.md, references/foo.md). Use `(none)` if the source was consulted but contributed no file content directly.
+- **Status:** `extracted`