feat(coding-agent): add --no-tools flag to disable built-in tools

Add --no-tools flag that allows starting pi without any built-in tools,
enabling extension-only tool setups (e.g., pi-ssh-remote).

- Add --no-tools flag to CLI args parsing
- Handle --tools '' (empty string) as equivalent to no tools
- Fix system prompt to not show READ-ONLY mode when no tools (extensions may provide write capabilities)
- Add tests for new flag and system prompt behavior

fixes #555

packages/coding-agent/src/main.ts

@@ -177,7 +177,15 @@ function buildSessionOptions(
 	}
 
 	// Tools
-	if (parsed.tools) {
+	if (parsed.noTools) {
+		// --no-tools: start with no built-in tools
+		// --tools can still add specific ones back
+		if (parsed.tools && parsed.tools.length > 0) {
+			options.tools = parsed.tools.map((name) => allTools[name]);
+		} else {
+			options.tools = [];
+		}
+	} else if (parsed.tools) {
 		options.tools = parsed.tools.map((name) => allTools[name]);
 	}