Added first-class PowerShell support on Windows

Improvements

• CorePlugin installs now preserve JSONC comments in configuration files
• CoreFixed OPENCODE_DISABLE_CLAUDE_CODE_PROMPT not being respected for project-level CLAUDE.md
• TerminalImproved variant modal behavior to be less intrusive
• TerminalAdded theme colors for dialog textarea placeholders
• CoreFixed plugin entrypoint resolution for paths without leading dot