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/assets/plugin-agent-claude.md

@@ -0,0 +1,8 @@
+---
+name: AGENT_NAME
+description: AGENT_DESCRIPTION
+tools: AGENT_TOOLS
+model: AGENT_MODEL
+---
+
+AGENT_SYSTEM_PROMPT

plugins/kyberforge/skills/write-agent/assets/plugin-agent-copilot.md

@@ -0,0 +1,14 @@
+<!-- HANDOFF: The following Claude Code fields were not ported — no Copilot CLI equivalent:
+     CLAUDE_ONLY_FIELDS_NOTE
+     Add Copilot-specific fields (target, disable-model-invocation, user-invocable) if needed.
+     Verify tool names against Copilot CLI tool aliases (codebase, search, edit/editFiles, etc.)
+     rather than Claude Code internal names (Read, Grep, Bash, etc.). -->
+---
+name: AGENT_NAME
+description: AGENT_DESCRIPTION
+tools:
+  - AGENT_TOOLS_COPILOT
+model: AGENT_MODEL_COPILOT
+---
+
+AGENT_SYSTEM_PROMPT

plugins/kyberforge/skills/write-agent/assets/subagent.md

@@ -0,0 +1,8 @@
+---
+name: AGENT_NAME
+description: AGENT_DESCRIPTION
+tools: AGENT_TOOLS
+model: AGENT_MODEL
+---
+
+AGENT_SYSTEM_PROMPT