feat(kyberforge): add agent-author skill for Claude Code and Copilot CLI agents

## Why

skill-author explicitly excludes agent definition files ("Do not use to author
agent definition files"). No factory skill existed to create or improve the
.md / .agent.md files that define Claude Code subagents and Copilot CLI agents
in a plugin, project, or user scope. This fills that gap.

## Implementation Notes

Single-root scaffold convention: new-agent.sh <name> <root> derives both
provider file paths from the root by convention — plugin scope (plugin.json
present) writes both files into <root>/agents/; non-plugin scope writes
.claude/agents/<name>.md and .github/agents/<name>.agent.md. This keeps
input minimal while always generating both provider files. See ADR-0015.

Routing is file-level (not directory-level like skill-author): neither file
exists → create flow; at least one exists → improve flow; scaffold is a
file-by-file no-op so retries are safe.

No companion agent-audit skill — inline validation in the close step covers
the simpler agent field contract. agent-audit is tracked as a follow-on.

## Impact

Closes the skill-author gap for agent definitions. Follow-ons tracked in
Gitea #11: agent-audit skill and --copilot-dest override flag for non-standard
Copilot project paths.

---
ADR: docs/adr/0015-agent-author-dual-provider-scaffold.md
Refs: #10
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

plugins/kyberforge/skills/agent-author/assets/README.md

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+# assets/
+
+## templates/
+
+Annotated agent definition templates copied by `scripts/new-agent.sh` when scaffolding a new agent.
+
+- **`claude-code.md`** — Claude Code agent definition template. Includes all supported frontmatter fields (required and optional) with inline guidance comments and `FILL IN:` placeholders. Notes which fields are silently ignored for plugin agents.
+- **`copilot.agent.md`** — Copilot CLI agent definition template. Uses Copilot-specific fields (`target`, `user-invocable`, `disable-model-invocation`, Copilot tool aliases). Explicitly excludes Claude Code-only fields.

plugins/kyberforge/skills/agent-author/assets/templates/claude-code.md

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+---
+# Claude Code agent definition
+# Fill in all FILL IN: placeholders. Remove or uncomment optional fields as needed.
+
+name: AGENT_NAME
+# Required. Lowercase letters and hyphens only. Must be unique within the scope.
+# Duplicate names are silently discarded — no warning is emitted.
+
+description: FILL IN: Action-first description of what this agent does and when to invoke it.
+# Required. The primary signal for autonomous delegation.
+# Start with a verb: "Reviews...", "Analyzes...", "Generates..."
+# Include "Use proactively" to trigger automatic invocation.
+# Be specific about the triggering condition and domain.
+# Example: "Reviews pull request diffs for security issues. Use proactively after code changes."
+
+# tools: Read Bash Grep
+# Optional. Space-separated allowlist. Omit to inherit all tools from parent.
+# Use Agent(type1,type2) to restrict which subagent types this agent can spawn.
+# Omit Agent entirely to prevent this agent from spawning subagents.
+# Never available to subagents regardless of tools field:
+#   AskUserQuestion, EnterPlanMode, ExitPlanMode, ScheduleWakeup
+
+# model: sonnet
+# Optional. Aliases: sonnet, opus, haiku, fable. Or full model ID.
+# Omit to inherit from the main session.
+# Resolution order: CLAUDE_CODE_SUBAGENT_MODEL env var → per-invocation param → this field → session model.
+
+# effort: medium
+# Optional. low / medium / high / xhigh / max. Overrides session effort level for this agent.
+
+# maxTurns: 20
+# Optional. Integer cap on agentic turns. Prevents runaway on bounded tasks.
+
+# memory: project
+# Optional. user / project / local. Enables cross-session MEMORY.md (first 200 lines loaded at startup).
+# Auto-enables Read/Write/Edit tools.
+
+# isolation: worktree
+# Optional. Set to "worktree" to run in an isolated temporary git worktree.
+# Auto-cleaned if no changes are made.
+
+# color: blue
+# Optional. UI color: red, blue, green, yellow, purple, orange, pink, cyan.
+
+# background: false
+# Optional. Set true to force background execution.
+
+# NOTE: hooks, mcpServers, and permissionMode are silently ignored for plugin agents.
+# Those fields only work in .claude/agents/ or ~/.claude/agents/.
+---
+
+FILL IN: System prompt body. Write as a direct role instruction.
+
+You are a FILL IN: role description. When invoked, FILL IN: primary action.
+
+## Inputs
+
+FILL IN: What inputs does this agent expect? (files, context, parameters)
+
+## Process
+
+FILL IN: Steps the agent takes. Be specific about ordering if it matters.
+
+## Output
+
+FILL IN: What does the agent produce? Format, location, structure.

plugins/kyberforge/skills/agent-author/assets/templates/copilot.agent.md

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+---
+# GitHub Copilot CLI agent definition
+# File extension MUST be .agent.md — a plain .md file is not picked up by Copilot.
+# Fill in all FILL IN: placeholders. Remove or uncomment optional fields as needed.
+
+name: AGENT_NAME
+# Required. Kebab-case identifier. Home-directory version wins on name collision.
+
+description: FILL IN: Action-first description of what this agent does and when to invoke it.
+# Required. Used by the runtime for automatic agent selection — quality matters.
+# Start with a verb: "Reviews...", "Analyzes...", "Generates..."
+# Example: "Reviews pull request diffs for security issues."
+
+# tools: ["read", "search", "edit"]
+# Optional. Array of tool names. Omit = all available tools. [] = no tools.
+# Copilot tool aliases (use these, not Claude Code names):
+#   execute   — run shell commands (aliases: shell, Bash, powershell)
+#   read      — read file contents (aliases: Read, NotebookRead)
+#   edit      — modify files (aliases: Edit, MultiEdit, Write, NotebookEdit)
+#   search    — search files (aliases: Grep, Glob)
+#   agent     — invoke sub-agents (aliases: custom-agent, Task)
+#   web       — web search and fetch (aliases: WebSearch, WebFetch)
+# For MCP tools: "server-name/tool-name" or "server-name/*"
+
+# target: github-copilot
+# Optional. Which runtime loads this file.
+#   vscode          — VS Code Copilot only
+#   github-copilot  — Copilot CLI / cloud agents only
+#   both (default)  — loaded by both runtimes
+
+# user-invocable: true
+# Optional. Set false to hide from manual invocation (auto-select only).
+
+# disable-model-invocation: false
+# Optional. Set true to require explicit user invocation; prevents auto-selection.
+
+# model: claude-sonnet-4-5
+# Optional. Model to run this agent on.
+
+# DO NOT include these Claude Code-only fields:
+#   maxTurns, isolation, memory, permissionMode, effort, hooks, mcpServers
+---
+
+FILL IN: System prompt body. Should match the Claude Code version — the agent's task is the same across providers.
+
+You are a FILL IN: role description. When invoked, FILL IN: primary action.
+
+## Inputs
+
+FILL IN: What inputs does this agent expect? (files, context, parameters)
+
+## Process
+
+FILL IN: Steps the agent takes. Be specific about ordering if it matters.
+
+## Output
+
+FILL IN: What does the agent produce? Format, location, structure.