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/extensions/file-trigger.ts

@@ -0,0 +1,41 @@
+/**
+ * File Trigger Extension
+ *
+ * Watches a trigger file and injects its contents into the conversation.
+ * Useful for external systems to send messages to the agent.
+ *
+ * Usage:
+ *   echo "Run the tests" > /tmp/agent-trigger.txt
+ */
+
+import * as fs from "node:fs";
+import type { ExtensionAPI } from "@mariozechner/pi-coding-agent";
+
+export default function (pi: ExtensionAPI) {
+	pi.on("session_start", async (_event, ctx) => {
+		const triggerFile = "/tmp/agent-trigger.txt";
+
+		fs.watch(triggerFile, () => {
+			try {
+				const content = fs.readFileSync(triggerFile, "utf-8").trim();
+				if (content) {
+					pi.sendMessage(
+						{
+							customType: "file-trigger",
+							content: `External trigger: ${content}`,
+							display: true,
+						},
+						{ triggerTurn: true }, // triggerTurn - get LLM to respond
+					);
+					fs.writeFileSync(triggerFile, ""); // Clear after reading
+				}
+			} catch {
+				// File might not exist yet
+			}
+		});
+
+		if (ctx.hasUI) {
+			ctx.ui.notify(`Watching ${triggerFile}`, "info");
+		}
+	});
+}