feat(agents): project-local agent overrides

loadAgents merges <cwd>/agents/*.md on top of the extension's baseline:
a repo agent wins on name collision and may add brand-new agents. The
unknown-agent error now names both searched directories.

README.md

@@ -137,7 +137,10 @@ registerCheck(def)            ← checks/*.ts self-register on load
   `cwd` (see `src/agent-runner.ts`), with the agent definition's system prompt
   and tool allowlist applied. Agent definitions are plain editable markdown in
   [`agents/`](./agents/) (frontmatter `name` + `allowedTools`, body = system
-  prompt) — tuning a sub-agent never needs TypeScript changes.
+  prompt) — tuning a sub-agent never needs TypeScript changes. A scanned
+  project can ship its own `agents/*.md` at its root: those are loaded as
+  overrides (repo agent wins on name collision), so teams can tune prompts or
+  add project-specific agents without touching the extension.
 - **Run-state** is a single JSON file at `<cwd>/.pygienium/run-state.json`
   (`src/run-state.ts`): per-check phase progress, captured findings/changes
   text, and recon status. `/pygienium-status`, `/pygienium-resume`, and