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/CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Added
+
+- `--no-tools` flag to disable all built-in tools, allowing extension-only tool setups ([#555](https://github.com/badlogic/pi-mono/issues/555))
+
 ## [0.38.0] - 2026-01-08
 
 ### Breaking Changes