slice 1.0.2: write_file tool with workspace-root path validation

[sausi-7]

Phase 1.0 demo slice · Difficulty: D2

Scope

write_file(path, content) tool. Path resolved against the demo workspace root; reject .., absolute paths, symlink escape.

Acceptance criteria

• Creates parent dirs as needed.
• Emits a file_changed event after a successful write.
• Unit tests for traversal / absolute / symlink-escape rejection.

Same code as full Phase 1 issue 1.1.6.