fix: Windows support for process signals and session persistence

[thoraxe]

Summary

• Use platform-specific process termination: kill() without arguments on Windows, SIGTERM/SIGKILL on Unix
• Add fallback for session file save when rename() fails on Windows
• Ensure storage directory initialization completes before save/load operations

Problem

On Windows:

1. process.kill('SIGTERM') throws 'Signals not supported on windows'
2. fs.rename() can fail due to file locks or cross-device moves

Solution

• Check process.platform === 'win32' and use appropriate termination method
• Catch rename errors and fall back to direct file write
• Track initialization state to prevent race conditions

Test plan

• Test session stop on Windows (no more signal errors)
• Test session persistence on Windows (fallback to direct write)
• Verify Unix behavior unchanged (SIGTERM/SIGKILL still works)

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