feat: add support for rules to define multiple or single selection

[ndizazzo]

Summary

Added a mode configuration option to rules that allows specifying how items can be selected: multi-select (default, allows any combination) or single-select (enforce one-at-a-time selection via radio buttons).

This enables rules with mutually exclusive alternatives like oh-my-opencode.yaml with three provider configs targeting the same file to prevent users from accidentally installing multiple conflicting options. The feature includes TUI radio button indicators, headless enforcement, and applies to all bundled rules (6 multi-select, 1 single-select).

Type of change

• Bug fix
• New feature
• Refactor / cleanup
• Documentation
• Other:

Testing

• npm test passes (178 tests, 0 failures — includes 5 new mode tests)
• npm run lint:agents outputs agent-lint: ok
• New tests added for new behaviour:
  • tests/load-config.test.js: 3 tests for mode normalization (default, explicit, invalid values)
  • tests/install-core.test.js: 2 tests for mode propagation to profiles

Checklist

• No "use strict" omitted from new .js files
• No empty catch {} blocks introduced
• No new runtime dependencies added
• CHANGELOG.md updated under [Unreleased] (user may want to do this)

Files Changed

Data layer (mode parsing & propagation):

• scripts/load-config.js — Parse mode from YAML, default to multi-select
• scripts/install-core.js — Propagate mode to profiles
• scripts/tui/ui/format.mjs — Propagate mode to tool groups
  TUI (selection enforcement & visual indicators):
• scripts/tui/App.mjs — Enforce single-select in headless mode, disk scan initialization, and keyboard handlers (depth 0 and 1)
• scripts/tui/components/SelectionScreen.mjs — Radio button indicators (•) / ( ) for single-select groups
  Rules (explicit mode declarations):
• rules/claude.yaml, codex.yaml, copilot.yaml, cursor.yaml, gemini.yaml, opencode.yaml — Added mode: multi-select
• rules/oh-my-opencode.yaml — Added mode: single-select

Documentation:

• README.md — New "Writing Rules" section covering schema, selection modes, and examples
• CONTRIBUTING.md — Updated test count, reference to Writing Rules
  Tests:
• tests/load-config.test.js — 3 new tests for mode normalization
• tests/install-core.test.js — 2 new tests for mode propagation, updated delta test rule with mode: single-select