Merge hooks and custom-tools into unified extensions system (#454)

Breaking changes:
- Settings: 'hooks' and 'customTools' arrays replaced with 'extensions'
- CLI: '--hook' and '--tool' flags replaced with '--extension' / '-e'
- API: HookMessage renamed to CustomMessage, role 'hookMessage' to 'custom'
- API: FileSlashCommand renamed to PromptTemplate
- API: discoverSlashCommands() renamed to discoverPromptTemplates()
- Directories: commands/ renamed to prompts/ for prompt templates

Migration:
- Session version bumped to 3 (auto-migrates v2 sessions)
- Old 'hookMessage' role entries converted to 'custom'

Structural changes:
- src/core/hooks/ and src/core/custom-tools/ merged into src/core/extensions/
- src/core/slash-commands.ts renamed to src/core/prompt-templates.ts
- examples/hooks/ and examples/custom-tools/ merged into examples/extensions/
- docs/hooks.md and docs/custom-tools.md merged into docs/extensions.md

New test coverage:
- test/extensions-runner.test.ts (10 tests)
- test/extensions-discovery.test.ts (26 tests)
- test/prompt-templates.test.ts

packages/coding-agent/examples/custom-tools/subagent/agents.ts

@@ -1,156 +0,0 @@
-/**
- * Agent discovery and configuration
- */
-
-import * as fs from "node:fs";
-import * as os from "node:os";
-import * as path from "node:path";
-
-export type AgentScope = "user" | "project" | "both";
-
-export interface AgentConfig {
-	name: string;
-	description: string;
-	tools?: string[];
-	model?: string;
-	systemPrompt: string;
-	source: "user" | "project";
-	filePath: string;
-}
-
-export interface AgentDiscoveryResult {
-	agents: AgentConfig[];
-	projectAgentsDir: string | null;
-}
-
-function parseFrontmatter(content: string): { frontmatter: Record<string, string>; body: string } {
-	const frontmatter: Record<string, string> = {};
-	const normalized = content.replace(/\r\n/g, "\n");
-
-	if (!normalized.startsWith("---")) {
-		return { frontmatter, body: normalized };
-	}
-
-	const endIndex = normalized.indexOf("\n---", 3);
-	if (endIndex === -1) {
-		return { frontmatter, body: normalized };
-	}
-
-	const frontmatterBlock = normalized.slice(4, endIndex);
-	const body = normalized.slice(endIndex + 4).trim();
-
-	for (const line of frontmatterBlock.split("\n")) {
-		const match = line.match(/^([\w-]+):\s*(.*)$/);
-		if (match) {
-			let value = match[2].trim();
-			if ((value.startsWith('"') && value.endsWith('"')) || (value.startsWith("'") && value.endsWith("'"))) {
-				value = value.slice(1, -1);
-			}
-			frontmatter[match[1]] = value;
-		}
-	}
-
-	return { frontmatter, body };
-}
-
-function loadAgentsFromDir(dir: string, source: "user" | "project"): AgentConfig[] {
-	const agents: AgentConfig[] = [];
-
-	if (!fs.existsSync(dir)) {
-		return agents;
-	}
-
-	let entries: fs.Dirent[];
-	try {
-		entries = fs.readdirSync(dir, { withFileTypes: true });
-	} catch {
-		return agents;
-	}
-
-	for (const entry of entries) {
-		if (!entry.name.endsWith(".md")) continue;
-		if (!entry.isFile() && !entry.isSymbolicLink()) continue;
-
-		const filePath = path.join(dir, entry.name);
-		let content: string;
-		try {
-			content = fs.readFileSync(filePath, "utf-8");
-		} catch {
-			continue;
-		}
-
-		const { frontmatter, body } = parseFrontmatter(content);
-
-		if (!frontmatter.name || !frontmatter.description) {
-			continue;
-		}
-
-		const tools = frontmatter.tools
-			?.split(",")
-			.map((t) => t.trim())
-			.filter(Boolean);
-
-		agents.push({
-			name: frontmatter.name,
-			description: frontmatter.description,
-			tools: tools && tools.length > 0 ? tools : undefined,
-			model: frontmatter.model,
-			systemPrompt: body,
-			source,
-			filePath,
-		});
-	}
-
-	return agents;
-}
-
-function isDirectory(p: string): boolean {
-	try {
-		return fs.statSync(p).isDirectory();
-	} catch {
-		return false;
-	}
-}
-
-function findNearestProjectAgentsDir(cwd: string): string | null {
-	let currentDir = cwd;
-	while (true) {
-		const candidate = path.join(currentDir, ".pi", "agents");
-		if (isDirectory(candidate)) return candidate;
-
-		const parentDir = path.dirname(currentDir);
-		if (parentDir === currentDir) return null;
-		currentDir = parentDir;
-	}
-}
-
-export function discoverAgents(cwd: string, scope: AgentScope): AgentDiscoveryResult {
-	const userDir = path.join(os.homedir(), ".pi", "agent", "agents");
-	const projectAgentsDir = findNearestProjectAgentsDir(cwd);
-
-	const userAgents = scope === "project" ? [] : loadAgentsFromDir(userDir, "user");
-	const projectAgents = scope === "user" || !projectAgentsDir ? [] : loadAgentsFromDir(projectAgentsDir, "project");
-
-	const agentMap = new Map<string, AgentConfig>();
-
-	if (scope === "both") {
-		for (const agent of userAgents) agentMap.set(agent.name, agent);
-		for (const agent of projectAgents) agentMap.set(agent.name, agent);
-	} else if (scope === "user") {
-		for (const agent of userAgents) agentMap.set(agent.name, agent);
-	} else {
-		for (const agent of projectAgents) agentMap.set(agent.name, agent);
-	}
-
-	return { agents: Array.from(agentMap.values()), projectAgentsDir };
-}
-
-export function formatAgentList(agents: AgentConfig[], maxItems: number): { text: string; remaining: number } {
-	if (agents.length === 0) return { text: "none", remaining: 0 };
-	const listed = agents.slice(0, maxItems);
-	const remaining = agents.length - listed.length;
-	return {
-		text: listed.map((a) => `${a.name} (${a.source}): ${a.description}`).join("; "),
-		remaining,
-	};
-}