docs(kyberforge): add skill-vs-agent decision criteria for Claude Code and Copilot

Fills a gap needed for the planned forge orchestrator skill: neither doc set
previously stated when to build a skill vs a subagent/custom agent. Sourced
via context7 against the same libraries already recorded in each sources.md.
Also confirms agentskills.io's spec is runtime-agnostic and defines no agent
concept, so it has no bearing on this decision by design.

plugins/kyberforge/docs/research/docs/agentskillsio/overview.md

@@ -4,6 +4,7 @@ source_keys:
   - agentskills-home
   - agentskills-spec
   - agentskills-quickstart
+  - agentskills-best-practices
 ---
 
 ## What Agent Skills is
@@ -30,6 +31,10 @@ Agents load skills in three stages:
 
 Full instructions load only when a task calls for them, so agents can keep many skills on hand with only a small context footprint.
 
+## Scope: no concept of "agent"
+
+The spec is runtime-agnostic and defines only the skill format — it does not define "agent," "subagent," or any delegation/orchestration concept, and neither the specification nor the best-practices guide contains criteria for choosing a skill over a separate agent process. Skill-vs-agent selection is a decision made by whichever runtime consumes the skill (see the Claude Code and GitHub Copilot decision-criteria docs for their respective answers), not something the Agent Skills format itself addresses.
+
 ## Canonical directory
 
 The canonical location for skills is `.agents/skills/` at the project root. Tool-specific locations (`.claude/skills/` for Claude Code, `~/.codex/skills/` for Codex) are thin adapters that map to this canonical path. Putting skills at `.agents/skills/` maximizes cross-tool portability.