refactor: improve type safety and ui consistency across extensions

[kostyay]

• Add TypeScript type checking to lint script with tsc --noEmit to catch type errors early in CI pipeline
• Update EditorTheme property names in answer.ts for consistency: selectedBg → selectedPrefix, matchHighlight → selectedText, itemSecondary → description, and add scrollInfo/noMatch properties
• Improve type safety in control.ts by explicitly casting union types and defensively handling content arrays that may not be arrays
• Add ThemeColor type annotations to replace magic strings in kt/index.ts and plan-ask.ts, improving type consistency across theme color usage
• Change notification levels from "success" to "info" in git-rebase-master.ts, sandbox/index.ts, and plan-ask.ts for semantic consistency
• Fix null safety issues with optional chaining guards in files.ts, loop.ts, and notify.ts event handlers
• Simplify select list UI in kt/index.ts by using string labels instead of complex objects with descriptions
• Remove unused searchable = true assignments from review.ts select lists (already enabled by default)
• Re-implement sliceByColumn locally in session-breakdown.ts since pi-tui no longer exports it
• Delete unused todos.ts extension (2,075 lines removed, reducing maintenance burden)
• Export ModeDisplay interface and MODE_DISPLAY constant from plan-ask.ts for reusability in other extensions
• Fix type inference in prompt-editor.ts and improve mode selection flow with proper type annotations
• Add comprehensive test coverage for plan-ask extension (248 lines of new tests)