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/evals/write-agent.yaml

@@ -0,0 +1,103 @@
+skill_name: write-agent
+
+trigger_tests:
+  - id: explicit-basic
+    name: Explicit trigger — basic invocation
+    query: "write an agent for code review"
+    should_trigger: true
+
+  - id: explicit-plugin
+    name: Explicit trigger — plugin agent
+    query: "add an agent to the security-tools plugin"
+    should_trigger: true
+
+  - id: implicit-isolation
+    name: Implicit trigger — isolated worker need
+    query: "I need a specialized worker that handles DB migrations in isolation"
+    should_trigger: true
+
+  - id: negative-role-skill
+    name: Negative — role skill (inline mode switch)
+    query: "Add an architect role to my session"
+    should_trigger: false
+
+  - id: negative-plugin-create
+    name: Negative — plugin scaffolding
+    query: "Create a new plugin called security-tools"
+    should_trigger: false
+
+  - id: negative-edit-existing
+    name: Negative — editing an existing agent
+    query: "Update the description on my existing code-reviewer agent"
+    should_trigger: false
+
+  - id: negative-skill-authoring
+    name: Negative — skill authoring inside a plugin
+    query: "Write a skill for my refactor plugin"
+    should_trigger: false
+
+output_tests:
+  - id: subagent-output-path
+    name: Standalone subagent written to core/agents/
+    type: deterministic
+    prompt: "Create a standalone subagent called db-migrator that runs database migrations in an isolated context"
+    expected_output: >
+      The skill produces a file at core/agents/db-migrator.md containing valid YAML frontmatter
+      with at minimum name and description fields, followed by a system prompt body.
+    assertions:
+      - "Output references the file path core/agents/db-migrator.md"
+      - "Output does not reference .claude/agents/ as the write target"
+      - "Generated file contains a --- frontmatter block with name: db-migrator"
+
+  - id: plugin-agent-dual-file
+    name: Plugin agent produces both .md and .agent.md
+    type: deterministic
+    prompt: "Add a cross-tool agent called security-scanner to the existing kyberforge plugin"
+    expected_output: >
+      The skill produces two files: plugins/kyberforge/agents/security-scanner.md (Claude Code)
+      and plugins/kyberforge/agents/security-scanner.agent.md (Copilot CLI). Both files share
+      an identical system prompt body.
+    assertions:
+      - "Output references plugins/kyberforge/agents/security-scanner.md"
+      - "Output references plugins/kyberforge/agents/security-scanner.agent.md"
+      - "Skill does not produce only one of the two files"
+
+  - id: claude-frontmatter-required-fields
+    name: Claude Code agent file contains required frontmatter fields
+    type: deterministic
+    prompt: "Write an agent for linting that reads files and reports violations"
+    expected_output: >
+      The generated .md file contains a YAML frontmatter block with at minimum name: and
+      description: fields. The description field contains delegation guidance — when Claude
+      should invoke this agent.
+    assertions:
+      - "Generated frontmatter contains name:"
+      - "Generated frontmatter contains description:"
+      - "The description value is not empty or a placeholder"
+
+  - id: copilot-handoff-comment
+    name: Copilot CLI .agent.md contains handoff comment for Claude-only fields
+    type: deterministic
+    prompt: "Add a plugin agent to the kyberforge plugin that uses permissionMode: bypassPermissions and isolation: worktree"
+    expected_output: >
+      The generated .agent.md file contains a comment at the top noting that permissionMode
+      and isolation are Claude Code-only fields with no Copilot CLI equivalent.
+    assertions:
+      - "The .agent.md file contains a HANDOFF comment or equivalent note"
+      - "The comment names at least one Claude-only field that was not ported"
+      - "The .agent.md frontmatter does not contain permissionMode or isolation fields"
+
+  - id: description-field-quality
+    name: LLM rubric — description field names specific tasks not just a role
+    type: llm-rubric
+    prompt: "Create a standalone subagent called api-tester that tests REST API endpoints for correctness and schema compliance"
+    expected_output: >
+      The description field in the generated agent file should specify delegation conditions
+      precisely enough that an orchestrator model can decide whether to invoke this agent
+      without ambiguity. It should name the task domain, the conditions for delegation,
+      and ideally one or more negative cases. A description that only says "Tests APIs" or
+      restates the agent name is insufficient.
+    assertions:
+      - "The description field mentions what kind of task triggers delegation to this agent"
+      - "The description field does not simply restate the agent name"
+      - "The description is specific enough to distinguish this agent from a general-purpose testing agent"