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-worktrees/references/README.md

@@ -0,0 +1,13 @@
+---
+source_keys:
+  - git-scm-worktree-docs
+---
+
+# References
+
+This directory contains provenance metadata and research sources for the `git-worktrees` skill.
+
+## Files
+
+- `sources.md` — Extracted research sources and their contributing documents
+- `worktrees.md` — Full `add` flag table, sparse-checkout setup, removable-media locking, remote-branch disambiguation, and the config key reference

plugins/git/skills/git-worktrees/references/sources.md

@@ -0,0 +1,16 @@
+---
+# Research sources referenced by this skill
+# Each entry documents where the skill's guidance came from.
+---
+
+## git-scm-worktree-docs
+
+**Description:** Git SCM official documentation for `git worktree` command — creating, listing, locking, moving, removing, pruning, and repairing linked worktrees; shared vs. per-worktree state; worktree-scoped config.
+
+**Source:** https://git-scm.com/docs/git-worktree
+
+- **Research doc:** plugins/git/docs/research/docs/git/worktrees.md (whole-document reference — covers `## Concept Overview`, `## Key Commands`, `## Workflow Patterns`, `## Common Gotchas`, `## Configuration`)
+
+**Contributing files:**
+- SKILL.md (Concept, Gotchas, Common Operations, Worked Examples, Return Format)
+- references/worktrees.md (full `add` flag table, sparse-checkout, removable media, remote disambiguation, configuration)

plugins/git/skills/git-worktrees/references/worktrees.md

@@ -0,0 +1,63 @@
+---
+topic: worktrees
+source_keys:
+  - git-scm-worktree-docs
+---
+
+## Full `add` flag table
+
+| Flag | Meaning |
+|---|---|
+| `-b <branch>` | Create and check out a new branch; fails if it exists |
+| `-B <branch>` | Like `-b` but resets the branch if it already exists |
+| `-d` / `--detach` | Detach HEAD; useful for throwaway experiments |
+| `--orphan` | Create empty unborn branch |
+| `--no-checkout` | Suppress initial checkout (for sparse-checkout setup) |
+| `--guess-remote` | Look for a matching remote-tracking branch by path basename |
+| `--lock [--reason <str>]` | Lock immediately on creation (atomic; avoids race vs. add-then-lock) |
+| `-f` / `--force` | Allow when branch is already checked out elsewhere |
+| `--relative-paths` | Link via relative paths (portable across moves) |
+
+Using `-` as `<commit-ish>` is shorthand for `@{-1}` (the branch checked out before the current one), e.g. `git worktree add <path> -`.
+
+## New unborn branch
+
+```bash
+git worktree add --orphan -b <branch> <path>
+```
+Creates an empty branch with no commits.
+
+## Sparse-checkout worktree
+
+Suppress the initial checkout to configure sparse-checkout first:
+```bash
+git worktree add --no-checkout ../sparse main
+cd ../sparse
+git sparse-checkout init --cone
+git sparse-checkout set src/
+git checkout main
+```
+
+## Worktree on removable media
+
+```bash
+git worktree add --lock --reason "external SSD" <path> <branch>
+git worktree unlock <path>  # when reconnected
+```
+
+## Remote-branch disambiguation
+
+```bash
+git worktree add <path> <remote>/<branch>
+```
+For ambiguous names across remotes, `checkout.defaultRemote` config disambiguates explicitly, or `--guess-remote` auto-matches by path basename (default controlled by `worktree.guessRemote` config). If a branch name matches multiple remotes during `worktree add` and neither is set, Git refuses rather than guessing.
+
+## Configuration
+
+| Key | Effect |
+|---|---|
+| `worktree.guessRemote` | Default for `--guess-remote` on `git worktree add` |
+| `worktree.useRelativePaths` | Default for `--relative-paths` on `git worktree add` (link via relative paths — portable across moves) |
+| `gc.worktreePruneExpire` | How long before stale worktree metadata is pruned by `git gc` |
+| `extensions.worktreeConfig` | Enable per-worktree config scope (`config.worktree` file) — see Gotchas in SKILL.md |
+| `checkout.defaultRemote` | Disambiguates which remote to use when a branch name matches multiple remotes during `worktree add` |