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/sdk/05-tools.ts

@@ -1,27 +1,28 @@
 /**
  * Tools Configuration
  *
- * Use built-in tool sets, individual tools, or add custom tools.
+ * Use built-in tool sets or individual tools.
  *
  * IMPORTANT: When using a custom `cwd`, you must use the tool factory functions
  * (createCodingTools, createReadOnlyTools, createReadTool, etc.) to ensure
  * tools resolve paths relative to your cwd, not process.cwd().
+ *
+ * For custom tools, see 06-extensions.ts - custom tools are now registered
+ * via the extensions system using pi.registerTool().
  */
 
 import {
-	bashTool, // read, bash, edit, write - uses process.cwd()
-	type CustomTool,
+	bashTool,
 	createAgentSession,
 	createBashTool,
-	createCodingTools, // Factory: creates tools for specific cwd
+	createCodingTools,
 	createGrepTool,
 	createReadTool,
 	grepTool,
-	readOnlyTools, // read, grep, find, ls - uses process.cwd()
+	readOnlyTools,
 	readTool,
 	SessionManager,
 } from "@mariozechner/pi-coding-agent";
-import { Type } from "@sinclair/typebox";
 
 // Read-only mode (no edit/write) - uses process.cwd()
 await createAgentSession({
@@ -53,38 +54,3 @@ await createAgentSession({
 	sessionManager: SessionManager.inMemory(),
 });
 console.log("Specific tools with custom cwd session created");
-
-// Inline custom tool (needs TypeBox schema)
-const weatherTool: CustomTool = {
-	name: "get_weather",
-	label: "Get Weather",
-	description: "Get current weather for a city",
-	parameters: Type.Object({
-		city: Type.String({ description: "City name" }),
-	}),
-	execute: async (_toolCallId, params) => ({
-		content: [{ type: "text", text: `Weather in ${(params as { city: string }).city}: 22°C, sunny` }],
-		details: {},
-	}),
-};
-
-const { session } = await createAgentSession({
-	customTools: [{ tool: weatherTool }],
-	sessionManager: SessionManager.inMemory(),
-});
-
-session.subscribe((event) => {
-	if (event.type === "message_update" && event.assistantMessageEvent.type === "text_delta") {
-		process.stdout.write(event.assistantMessageEvent.delta);
-	}
-});
-
-await session.prompt("What's the weather in Tokyo?");
-console.log();
-
-// Merge with discovered tools from cwd/.pi/tools and ~/.pi/agent/tools:
-// const discovered = await discoverCustomTools();
-// customTools: [...discovered, { tool: myTool }]
-
-// Or add paths without replacing discovery:
-// additionalCustomToolPaths: ["/extra/tools"]