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.

src/agent-runner.ts

@@ -18,6 +18,7 @@
 import { mkdir, writeFile } from "node:fs/promises";
 import { dirname, isAbsolute, join } from "node:path";
 import type { AgentSessionEvent } from "@earendil-works/pi-coding-agent";
+import { loadAgents, extensionRoot, type AgentDef } from "./agents.js";
 import type { ToolCallEntry } from "./phases.js";
 
 export interface AgentTaskOptions {
@@ -74,13 +75,19 @@ export function resetAgentRunner(): void {
 export async function defaultAgentRunner(
 	opts: AgentTaskOptions,
 ): Promise<AgentRunResult> {
-	const agent =
-		opts.agent ?? (await loadAgents({ cwd: opts.cwd })).get(opts.agentName);
+	const agents = await loadAgents({ cwd: opts.cwd });
+	const agent = opts.agent ?? agents.get(opts.agentName);
 	if (!agent) {
+		const names = [...agents.keys()];
 		return {
 			ok: false,
 			text: "",
-			error: `Unknown agent definition: "${opts.agentName}". Add agents/${opts.agentName}.md.`,
+			toolCalls: [],
+			error:
+				`Unknown agent definition: "${opts.agentName}". ` +
+				`Available agents: ${names.length > 0 ? names.join(", ") : "(none loaded — check agents/ directories exist)"}. ` +
+				`Searched ${extensionRoot()}/agents/ (extension) and ${opts.cwd}/agents/ (project-local). ` +
+				`Add agents/${opts.agentName}.md to either location.`,
 		};
 	}