fix(situla): support Windows config writes

[dajiaohuang]

Summary

• construct Situla's atomic configuration filename with the platform-aware path basename helper
• avoid asserting POSIX permission bits on Windows while retaining those checks on supported filesystems
• verify the atomic writer leaves only the final config.json

Fixes #253

Tests

• npm test (81 passed)
• npm run typecheck
• npm run build
• uvx pre-commit run --show-diff-on-failure --color=never --files python/01-tutorials/04-agentkit-tools/situla/src/config.ts python/01-tutorials/04-agentkit-tools/situla/test/config.test.ts