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/git/skills/git-commits/references/sources.md

@@ -0,0 +1,40 @@
+---
+topic: commits
+source_keys:
+  - conventional-commits-spec
+  - commitlint-config-conventional
+  - org-commit-conventions
+  - context7-git-htmldocs
+---
+
+# Research Sources for git:commits Skill
+
+Sources extracted from the git plugin research phase. Only sources that directly informed this skill are listed; sibling skills (git:branches, git:history, git:remotes, etc.) have their own sources.md.
+
+## conventional-commits-spec
+
+- **Description:** Conventional Commits Specification (v1.0.0) — message format, types, breaking changes, footer rules
+- **Research doc:** plugins/git/docs/research/docs/git/commits.md § "Conventional Commits Specification (v1.0.0)"
+- **Contributing files:** SKILL.md, references/conventional-commits-spec.md
+- **Status:** extracted
+
+## commitlint-config-conventional
+
+- **Description:** commitlint config-conventional preset — validation constraints (max 100 chars header, no trailing periods, lowercase type, 11-type set enforcement)
+- **Research doc:** plugins/git/docs/research/docs/git/commits.md § "commitlint Constraints (`config-conventional`)"
+- **Contributing files:** SKILL.md, references/conventional-commits-spec.md
+- **Status:** extracted
+
+## org-commit-conventions
+
+- **Description:** Organization commit message body template and git conventions (atomic commits, no `--no-verify`, no force-push main/master, `rtk git` wrapper) — content fully embedded in this skill; the org's `core/instructions/git.md` and `core/instructions/commits.md` are provenance only and are not a live dependency
+- **Research doc:** core/instructions/commits.md, core/instructions/git.md (org convention, not part of the plugin's research corpus)
+- **Contributing files:** SKILL.md, references/commit-template.md
+- **Status:** extracted
+
+## context7-git-htmldocs
+
+- **Description:** Official Git HTML documentation — `git commit --squash`/`--fixup` and `git rebase --autosquash` flag semantics
+- **Research doc:** plugins/git/docs/research/docs/git/cli-reference.md
+- **Contributing files:** SKILL.md
+- **Status:** extracted