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>

docs/adr/0005-agent-author-dual-provider-scaffold.md

@@ -39,3 +39,8 @@ separate single-provider skill, adding complexity with no benefit.
 - The file-by-file no-op in the script (skip existing files rather than
   overwriting) means partial state — one provider file exists, the other does not —
   is handled by routing in the skill body, not in the script.
+
+**Update (ADR-0010):** the `agents/sources.md` path above is superseded. The provenance
+file now lives at `<plugin-root>/sources.md`, outside the `agents/` directory, because
+`claude plugin validate --strict` auto-discovers every `.md` under `agents/` as an agent
+requiring frontmatter. See ADR-0010 for the empirical finding and rationale.