Add config validation command

[trick77]

Summary

• add opencode-presets validate [config|tui|all]
• validate OPENCODE_CONFIG and OPENCODE_TUI_CONFIG against their OpenCode schemas
• skip missing files in all mode and print labeled where/what/detail issues for invalid files
• reuse labeled schema issue formatting across install/remove validation output
• bump package version to 0.8.6

Test Plan

• npm test