feat(coding-agent): add Claude Code marketplace plugin discovery

Add discovery provider that loads skills, commands, hooks, tools, and
agents from ~/.claude/plugins/cache/ based on installed_plugins.json.

- Add parseClaudePluginsRegistry() and listClaudePluginRoots() helpers
- Create claude-plugins.ts provider with priority 70 (below claude.ts)
- Register provider for skills, slash-commands, hooks, and tools
- Integrate plugin agents into task/discovery.ts

Closes #48

packages/coding-agent/src/task/discovery.ts

@@ -12,9 +12,11 @@
  * Agent files use markdown with YAML frontmatter.
  */
 import * as fs from "node:fs/promises";
+import * as os from "node:os";
 import * as path from "node:path";
 import { logger } from "@oh-my-pi/pi-utils";
 import { findAllNearestProjectConfigDirs, getConfigDirs } from "../config";
+import { listClaudePluginRoots } from "../discovery/helpers";
 import { loadBundledAgents, parseAgent } from "./agents";
 import type { AgentDefinition, AgentSource } from "./types";
 
@@ -85,6 +87,13 @@ export async function discoverAgents(cwd: string): Promise<DiscoveryResult> {
 		if (user) orderedDirs.push({ dir: user.path, source: "user" });
 	}
 
+	// Load agents from Claude Code marketplace plugins
+	const { roots: pluginRoots } = await listClaudePluginRoots(os.homedir());
+	for (const plugin of pluginRoots) {
+		const agentsDir = path.join(plugin.path, "agents");
+		orderedDirs.push({ dir: agentsDir, source: "user" });
+	}
+
 	const seen = new Set<string>();
 	const loadedAgents = (await Promise.all(orderedDirs.map(({ dir, source }) => loadAgentsFromDir(dir, source))))
 		.flat()