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/META.md

@@ -0,0 +1,13 @@
+```yaml
+version: "1.0"
+updated: 2026-06-21
+
+when: invoked by explicit trigger ("write an agent for X", "create a subagent that does Y", "add an agent to the Z plugin") or implicit request to author a Claude Code subagent or GitHub Copilot CLI plugin agent definition file
+
+# source: omitted — self-authored original; no upstream content adopted
+
+references:
+  - https://code.claude.com/docs/en/sub-agents
+  - https://code.claude.com/docs/en/plugins-reference
+  - https://docs.github.com/en/copilot/reference/custom-agents-configuration
+```