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/references/deployment-modes.md

@@ -0,0 +1,63 @@
+---
+source_keys:
+  - context7-websites-code-claude
+  - claude-code-plugins-docs
+  - claude-code-subagents-docs
+  - github-custom-agents-configuration
+  - github-cli-plugin-reference
+---
+
+# Agent Deployment Modes
+
+Agent definitions deploy at three scopes and behave differently at each. The scope determines which fields are honoured, where files must live, and what identifiers users invoke.
+
+## Scope hierarchy and precedence
+
+| Scope | Claude Code path | Copilot CLI path | Who it affects |
+|---|---|---|---|
+| User | `~/.claude/agents/` | `~/.copilot/agents/` | All sessions for this user |
+| Project | `.claude/agents/` | `.github/agents/` or `.copilot/agents/` | This repo only |
+| Plugin | `<plugin>/agents/` | `<plugin>/agents/` | Sessions with the plugin installed |
+
+When the same agent `name` appears at multiple scopes, **user scope wins over project scope wins over plugin scope** in Claude Code. In Copilot CLI, repo-level agents override enterprise and org-level; home-directory (user) agents override repo-level on name collision.
+
+## Plugin scope restrictions
+
+Plugin agents (agents shipped inside a plugin's `agents/` directory) have fewer capabilities than project or user agents:
+
+**Claude Code — silently ignored for plugin agents:**
+- `hooks` — per-agent event handlers; copy the agent to `.claude/agents/` to use these
+- `mcpServers` — inline MCP server definitions; not available in plugin context
+- `permissionMode` — silently ignored; the parent session's permission mode applies
+
+**Copilot CLI — unsupported or ignored:**
+- `mcp-servers` — cloud agents only; ignored in VS Code and most IDEs
+- `metadata` — key-value annotations; ignored in VS Code
+
+## Scoped identifiers (Claude Code plugin agents only)
+
+Plugin agents in **subdirectories** get compound identifiers:
+
+```
+plugins/my-plugin/agents/review/security.md  →  my-plugin:review:security
+```
+
+Users must invoke with `@agent-my-plugin:review:security`. **Keep agents flat in `agents/` to avoid this** — subdirectory nesting is rarely worth the UX cost.
+
+At project and user scope, subdirectory path does not affect the agent's name.
+
+## Cache isolation
+
+When a plugin is installed, its directory is copied to a cache. **Any path that leaves the agent's plugin directory breaks post-install.** Agent definition files must be self-contained — they cannot reference scripts, templates, or shared files outside the plugin.
+
+Agents at project or user scope are read directly from disk; cache isolation does not apply.
+
+## Copilot CLI path conventions
+
+| Scope | Expected path | Notes |
+|---|---|---|
+| User | `~/.copilot/agents/<name>.agent.md` | Home directory |
+| Project | `.github/agents/<name>.agent.md` | Standard; also `.copilot/agents/` |
+| Plugin | `<plugin>/agents/<name>.agent.md` | Inside plugin root |
+
+The `.agent.md` extension is **mandatory** — Copilot CLI does not pick up plain `.md` files in the `agents/` directory.