refactor(skill): decouple store from Box runtime

[huanghuoguoguo]

Closes langbot-app/LangBot#2410

Summary

• Move SkillStore out of the Box domain so Core can activate and read Skill resources without a sandbox.
• Remove Skill RPC actions, Box client methods, Box Runtime Skill state, and the temporary mixed-version protocol bridge.
• Keep Box limited to generic read-only mount admission and reject retired Skill-aware payload fields.
• Replace full-package content SHA-256 scans with a stat-v1 metadata change token and one revision scan per resource operation.
• Raise the managed read-only mount limit from 64 to 256.

Compatibility boundary

Core and the Box Runtime SDK are upgraded as one release unit. No old-Core/new-Box or new-Core/old-SDK wire compatibility layer is retained.

Validation

• 463 Box and SkillStore tests passed.
• Ruff checks passed for changed SDK files.
• Action protocol consistency check passed.