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

scripts/check-manifests.sh

@@ -1,8 +1,15 @@
 #!/usr/bin/env bash
 set -euo pipefail
 
-# Validates that all paths referenced in marketplace.json and plugin.json manifests
-# resolve to existing files or directories. Run from repo root or pass REPO_ROOT as arg.
+# Validates that marketplace.json's local plugin entries resolve to a real directory
+# containing a .claude-plugin/plugin.json. Run from repo root or pass REPO_ROOT as arg.
+#
+# Per ADR-0015, apm.yml is the authoring source and .claude-plugin/plugin.json is
+# compiled output with no skills/hooks/mcpServers/agents pointer fields (apm's plugin.json
+# builder deliberately omits them -- Claude Code auto-discovers those convention
+# directories, so listing them would be redundant/invalid). This script no longer checks
+# those fields; that's now scripts/sync-plugin-content.sh --check's job (drift between
+# .apm/ and the flat plugin-root mirror), wired as its own pre-push hook.
 
 REPO_ROOT="${1:-$(git rev-parse --show-toplevel 2>/dev/null || pwd)}"
 FAIL=0
@@ -39,33 +46,11 @@ for ((i = 0; i < plugin_count; i++)); do
     continue
   fi
 
-  manifest="$plugin_dir/plugin.json"
+  manifest="$plugin_dir/.claude-plugin/plugin.json"
   if [[ ! -f "$manifest" ]]; then
-    err "plugin '$name': plugin.json not found in $source"
+    err "plugin '$name': .claude-plugin/plugin.json not found in $source"
     continue
   fi
-
-  # Check skills directories
-  skill_count=$(jq '.skills | if . then length else 0 end' "$manifest")
-  for ((s = 0; s < skill_count; s++)); do
-    skill_path=$(jq -r ".skills[$s]" "$manifest")
-    full_path="$plugin_dir/$skill_path"
-    full_path="${full_path%/}"
-    if [[ ! -d "$full_path" ]]; then
-      err "plugin '$name': skills path not found: $skill_path"
-    fi
-  done
-
-  # Check file references (hooks, mcpServers, agents)
-  for field in hooks mcpServers agents; do
-    ref=$(jq -r ".${field} // empty" "$manifest")
-    [[ -z "$ref" ]] && continue
-    full_path="$plugin_dir/$ref"
-    full_path="${full_path%/}"
-    if [[ ! -e "$full_path" ]]; then
-      err "plugin '$name': $field path not found: $ref"
-    fi
-  done
 done
 
 if [[ $FAIL -gt 0 ]]; then