feat(skills): add write-agent factory skill for cross-tool agent authoring

Adds write-agent to plugins/kyberforge/skills/ — a factory skill parallel
to write-skill that authors Claude Code subagent definitions and cross-tool
plugin agents (Claude Code + GitHub Copilot CLI two-file pattern).

Includes research references (claude-code-agents.md, copilot-cli-agents.md,
cross-compat.md), three asset templates (subagent, plugin-agent-claude,
plugin-agent-copilot), eval coverage, and CATEGORIES.md updated to register
write-agent in the factory category per the conflict check finding.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

plugins/kyberforge/skills/write-agent/references/sources.md

@@ -0,0 +1,36 @@
+# Sources
+
+## claude-code-docs-subagents
+
+- **URL:** https://code.claude.com/docs/en/sub-agents
+- **Description:** Official Claude Code documentation on creating and configuring custom subagents — all frontmatter fields, scope priority, tool restrictions, permission modes.
+- **Contributing files:** claude-code-agents.md, cross-compat.md
+- **Status:** `extracted`
+
+## claude-code-docs-plugins
+
+- **URL:** https://code.claude.com/docs/en/plugins-reference
+- **Description:** Official Claude Code plugin reference — agent fields available in plugin context and security restrictions (hooks, mcpServers, permissionMode ignored).
+- **Contributing files:** claude-code-agents.md, cross-compat.md
+- **Status:** `extracted`
+
+## github-copilot-custom-agents-config
+
+- **URL:** https://docs.github.com/en/copilot/reference/custom-agents-configuration
+- **Description:** GitHub Copilot custom agents configuration reference — all frontmatter fields, tools field format, target field, mcp-servers.
+- **Contributing files:** copilot-cli-agents.md, cross-compat.md
+- **Status:** `extracted`
+
+## github-blog-copilot-cli-agents
+
+- **URL:** https://github.blog/ai-and-ml/github-copilot/from-one-off-prompts-to-workflows-how-to-use-custom-agents-in-github-copilot-cli/
+- **Description:** GitHub blog post on Copilot CLI custom agents — .agent.md format, tool names, invocation pattern.
+- **Contributing files:** copilot-cli-agents.md
+- **Status:** `extracted`
+
+## plugin-marketplace-architecture
+
+- **URL:** /root/ai-development/plugins/kyberforge/docs/plugin-marketplace-architecture.md
+- **Description:** Repo-internal reference on Claude Code vs Copilot CLI divergence, two-file agent pattern, plugin constraints.
+- **Contributing files:** cross-compat.md
+- **Status:** `extracted`