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/forge/references/sources.md

@@ -0,0 +1,81 @@
+# Sources
+
+## claude-code-subagents-docs
+
+- **URL:** https://code.claude.com/docs/en/sub-agents
+- **Research doc:** plugins/kyberforge/docs/research/docs/claude-code-plugins/sources.md
+- **Description:** Official Claude Code subagent reference — definition format, all frontmatter fields, scope priority, built-in agents, CLI flags, environment variables, known limitations. Grounds Step 3's fork-vs-inline invocation logic: fork inherits full conversation history via `/fork` or `subagent_type: "fork"`, is not a declarable frontmatter field on any agent definition, cannot be nested (a fork cannot spawn another fork), and is a caller-side invocation choice rather than a property of the artifact being routed to.
+- **Contributing files:** SKILL.md
+- **Status:** `extracted`
+
+## context7-websites-code-claude
+
+- **URL:** context7:/websites/code_claude
+- **Research doc:** plugins/kyberforge/docs/research/docs/claude-code-plugins/sources.md
+- **Description:** Official Claude Code documentation site indexed by Context7 — confirms the `context: fork` skill-level frontmatter field means isolated/fresh execution, the opposite of what the `/fork` subagent command does (inherits conversation). Informs the Gotchas entry warning against conflating the two.
+- **Contributing files:** SKILL.md
+- **Status:** `extracted`
+
+## claude-code-plugins-docs
+
+- **URL:** https://code.claude.com/docs/en/plugins
+- **Research doc:** plugins/kyberforge/docs/research/docs/claude-code-plugins/sources.md
+- **Description:** Official Claude Code plugin authoring guide — plugin structure, manifest fields, loading methods, skill namespacing, agent activation, marketplace submission. Background context for Step 2's plugin/marketplace rows; no forge-specific content drawn directly from it beyond that.
+- **Contributing files:** (none)
+- **Status:** `extracted`
+
+## agentskills-spec
+
+- **URL:** https://agentskills.io/specification.md
+- **Research doc:** plugins/kyberforge/docs/research/docs/agentskillsio/sources.md
+- **Description:** Complete SKILL.md format specification — confirms `assets/`, `references/`, and `scripts/` are warranted only by the bulk/reusability of supporting content (large reference material, executable code, templates), not by a skill's category. forge has none of that bulk, so a lean SKILL.md-plus-provenance-file shape is spec-legitimate; the `references/sources.md` in this directory exists for this repo's own provenance-chain convention (see `CONTEXT.md`), not because the spec requires it.
+- **Contributing files:** SKILL.md
+- **Status:** `extracted`
+
+## agentskills-home
+
+- **URL:** https://agentskills.io/home.md
+- **Research doc:** plugins/kyberforge/docs/research/docs/agentskillsio/sources.md
+- **Description:** Agent Skills overview — not drawn on directly for forge; listed for provenance completeness against the agentskillsio research doc.
+- **Contributing files:** (none)
+- **Status:** `extracted`
+
+## agentskills-best-practices
+
+- **URL:** https://agentskills.io/skill-creation/best-practices.md
+- **Research doc:** plugins/kyberforge/docs/research/docs/agentskillsio/sources.md
+- **Description:** Best practices for skill creators — not drawn on directly for forge; listed for provenance completeness against the agentskillsio research doc.
+- **Contributing files:** (none)
+- **Status:** `extracted`
+
+## agentskills-optimizing-descriptions
+
+- **URL:** https://agentskills.io/skill-creation/optimizing-descriptions.md
+- **Research doc:** plugins/kyberforge/docs/research/docs/agentskillsio/sources.md
+- **Description:** How to test and improve skill descriptions for triggering accuracy — not drawn on directly for forge; listed for provenance completeness against the agentskillsio research doc.
+- **Contributing files:** (none)
+- **Status:** `extracted`
+
+## agentskills-evaluating-skills
+
+- **URL:** https://agentskills.io/skill-creation/evaluating-skills.md
+- **Research doc:** plugins/kyberforge/docs/research/docs/agentskillsio/sources.md
+- **Description:** Eval-driven skill quality improvement — not drawn on directly for forge; listed for provenance completeness against the agentskillsio research doc.
+- **Contributing files:** (none)
+- **Status:** `extracted`
+
+## agentskills-using-scripts
+
+- **URL:** https://agentskills.io/skill-creation/using-scripts.md
+- **Research doc:** plugins/kyberforge/docs/research/docs/agentskillsio/sources.md
+- **Description:** Using scripts in skills — not applicable to forge (no scripts/ directory); listed for provenance completeness against the agentskillsio research doc.
+- **Contributing files:** (none)
+- **Status:** `extracted`
+
+## agentskills-quickstart
+
+- **URL:** https://agentskills.io/skill-creation/quickstart.md
+- **Research doc:** plugins/kyberforge/docs/research/docs/agentskillsio/sources.md
+- **Description:** Step-by-step first-skill walkthrough — not drawn on directly for forge; listed for provenance completeness against the agentskillsio research doc.
+- **Contributing files:** (none)
+- **Status:** `extracted`