Add before/after session events with cancellation support

- Merge branch event into session with before_branch/branch reasons
- Add before_switch, before_clear, shutdown reasons
- before_* events can be cancelled with { cancel: true }
- Update RPC commands to return cancelled status
- Add shutdown event on process exit
- New example hooks: confirm-destructive, dirty-repo-guard, auto-commit-on-exit

fixes #278

packages/coding-agent/examples/hooks/dirty-repo-guard.ts

@@ -0,0 +1,59 @@
+/**
+ * Dirty Repo Guard Hook
+ *
+ * Prevents session changes when there are uncommitted git changes.
+ * Useful to ensure work is committed before switching context.
+ */
+
+import type { HookAPI } from "@mariozechner/pi-coding-agent/hooks";
+
+export default function (pi: HookAPI) {
+	pi.on("session", async (event, ctx) => {
+		// Only guard destructive actions
+		if (
+			event.reason !== "before_clear" &&
+			event.reason !== "before_switch" &&
+			event.reason !== "before_branch"
+		) {
+			return;
+		}
+
+		// Check for uncommitted changes
+		const { stdout, code } = await ctx.exec("git", ["status", "--porcelain"]);
+
+		if (code !== 0) {
+			// Not a git repo, allow the action
+			return;
+		}
+
+		const hasChanges = stdout.trim().length > 0;
+		if (!hasChanges) {
+			return;
+		}
+
+		if (!ctx.hasUI) {
+			// In non-interactive mode, block by default
+			return { cancel: true };
+		}
+
+		// Count changed files
+		const changedFiles = stdout.trim().split("\n").filter(Boolean).length;
+
+		const action =
+			event.reason === "before_clear"
+				? "clear session"
+				: event.reason === "before_switch"
+					? "switch session"
+					: "branch";
+
+		const choice = await ctx.ui.select(
+			`You have ${changedFiles} uncommitted file(s). ${action} anyway?`,
+			["Yes, proceed anyway", "No, let me commit first"],
+		);
+
+		if (choice !== "Yes, proceed anyway") {
+			ctx.ui.notify("Commit your changes first", "warning");
+			return { cancel: true };
+		}
+	});
+}