chore: pin SmallOS v1.2.0 release

[MikiEEE]

Summary

• replace the moving SmallOS master checkout with the v1.2.0 GitHub release tag
• use a standard direct VCS requirement instead of an editable dependency
• update install, development, platform, and release documentation

Validation

• verified the v1.2.0 tag resolves to the previously validated SmallOS b3b0e17 commit
• installed requirements.txt into an isolated target and confirmed SmallPackage 1.2.0
• full SmallServer suite: 181 tests passed against the tagged SmallOS source
• compileall and git diff --check passed