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/status-line.ts

@@ -0,0 +1,40 @@
+/**
+ * Status Line Extension
+ *
+ * Demonstrates ctx.ui.setStatus() for displaying persistent status text in the footer.
+ * Shows turn progress with themed colors.
+ */
+
+import type { ExtensionAPI } from "@mariozechner/pi-coding-agent";
+
+export default function (pi: ExtensionAPI) {
+	let turnCount = 0;
+
+	pi.on("session_start", async (_event, ctx) => {
+		const theme = ctx.ui.theme;
+		ctx.ui.setStatus("status-demo", theme.fg("dim", "Ready"));
+	});
+
+	pi.on("turn_start", async (_event, ctx) => {
+		turnCount++;
+		const theme = ctx.ui.theme;
+		const spinner = theme.fg("accent", "●");
+		const text = theme.fg("dim", ` Turn ${turnCount}...`);
+		ctx.ui.setStatus("status-demo", spinner + text);
+	});
+
+	pi.on("turn_end", async (_event, ctx) => {
+		const theme = ctx.ui.theme;
+		const check = theme.fg("success", "✓");
+		const text = theme.fg("dim", ` Turn ${turnCount} complete`);
+		ctx.ui.setStatus("status-demo", check + text);
+	});
+
+	pi.on("session_switch", async (event, ctx) => {
+		if (event.reason === "new") {
+			turnCount = 0;
+			const theme = ctx.ui.theme;
+			ctx.ui.setStatus("status-demo", theme.fg("dim", "Ready"));
+		}
+	});
+}