fix(agents): relocate provenance sources.md outside agents/ dir

`claude plugin validate --strict` auto-discovers every .md under a
plugin's agents/ directory as an agent requiring frontmatter, so the
provenance file there always needs fake agent frontmatter to pass
validation. Confirmed empirically that an explicit `agents` manifest
array can't suppress this discovery. Move the file to <plugin-root>/
sources.md instead, and update agent-author/agent-audit accordingly.

Adds ADR-0010, partially superseding ADR-0005's `agents/sources.md`
convention. Fixes #63.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

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

@@ -35,7 +35,7 @@ description: FILL IN: Action-first description of what this agent does and when
 
 <!-- source_keys:
        - slug-name
-     Development-only. Add when research sources informed this agent (slugs must match agents/sources.md).
+     Development-only. Add when research sources informed this agent (slugs must match sources.md at the plugin root).
      Omit when no research was used. Not a Copilot runtime field — silently ignored. -->
 
 <!-- DO NOT include these Claude Code-only fields: