fix(plugin): add OpenClaw state dir fallback

[YOMXXX]

Summary

• add an ESM-safe helper for resolving the OpenClaw state directory
• fall back to ~/.openclaw when api.runtime.state is unavailable during plugin registration
• reuse the same resolved state dir for plugin data and CLI registration
• keep the npm tarball under Size Guard by publishing built runtime artifacts only

Fixes #78.

This is intended as a safer alternative to #79: it avoids bare require("os") in the ESM source path and adds a focused unit test for the fallback behavior.

Verification

• npm test
• npm run build
• npm pack --dry-run (package size: 340.2 kB, unpacked: 1.3 MB)