feat(kyberforge): retarget agent-author to plugin-scope .apm/agents/

Plugin scope now authors a single vendor-neutral .apm/agents/<name>.agent.md
file (name/description/model/body only) instead of a Claude Code + Copilot
CLI pair, per ADR-0016: apm compile has no per-target field integrator, so
tools: and all Claude-only fields (isolation/maxTurns/effort/memory/
permissionMode) are omitted entirely rather than shipping a value that's
guaranteed wrong on one harness. Scope detection switches from a flat
plugin.json check to the same apm.yml+type: walk-up agent-audit's
validate.sh already uses. Project scope and user scope are unaffected —
both keep the existing dual-file pair model.

Refs: #89

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

@@ -0,0 +1,53 @@
+---
+<!-- Vendor-neutral APM agent definition (plugin/APM scope).
+     Path: <package-root>/.apm/agents/<name>.agent.md — one file, no counterpart.
+     `apm compile` copies this frontmatter verbatim to BOTH the Claude Code and
+     Copilot CLI targets — there is no per-target field integrator. Claude's
+     `tools:` (space-separated string) and Copilot's `tools:` (alias list) are
+     incompatible vocabularies, and Claude-only fields (isolation, maxTurns,
+     effort, memory, permissionMode) have no Copilot equivalent. A value correct
+     for one harness is guaranteed wrong on the other, so this scope carries
+     ONLY the three fields below — full stop (see ADR-0016).
+
+     Do NOT add: tools, isolation, maxTurns, effort, memory, permissionMode,
+     disallowedTools, skills, color, initialPrompt, background, hooks, or
+     mcpServers. Omitting `tools` means inherit-all-tools on both harnesses,
+     which is never wrong.
+
+     Fill in all FILL IN: placeholders. Delete template comments before shipping. -->
+
+name: AGENT_NAME
+<!-- Required. Lowercase letters and hyphens only. Must be unique within the scope. -->
+
+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..."
+     Be specific about the triggering condition and expertise domain.
+     Example: "Reviews pull request diffs for security issues. Use proactively after code changes." -->
+
+<!-- model: sonnet
+     Optional. Aliases: sonnet, opus, haiku, fable. Or full model ID.
+     Omit to inherit the runtime default on whichever harness compiles this file. -->
+
+<!-- source_keys:
+       - slug-name
+     Development-only. Add when research sources informed this agent (slugs must match
+     sources.md at the package root — same directory as apm.yml).
+     Omit when no research was used. Not a runtime field — silently ignored by both harnesses. -->
+---
+
+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.