feat: add append mode to file_write tool

[xlyoung]

Problem

The file_write tool hardcodes open(path, 'w'), always overwriting the file. The documentation at strandsagents.com claims append mode is supported, but the implementation doesn't support it.

Fix

Add optional mode parameter to file_write:

• mode='w' (default): overwrite file — existing behavior preserved
• mode='a': append content to existing file

Changes

1. TOOL_SPEC: Add mode parameter with enum ['w', 'a'] and default 'w'
2. Function: Read mode from tool input, use it in open(path, mode)
3. Success message: Reflect whether content was 'written to' or 'appended to'

Testing

All 9 file_write tests pass with no changes needed.

Fixes #344