refactor: modularize install.js into per-runtime modules

[amanape]

What

Breaks bin/install.js (2464 lines) into focused, per-runtime modules under bin/lib/ — core.js, codex.js, opencode.js, gemini.js, claude.js — reducing install.js to a thin orchestrator.

Why

The monolithic installer handles 4 runtimes (Claude Code, OpenCode, Gemini, Codex) with distinct conversion, config, and install/uninstall logic all in one file. This makes it hard to test, maintain, and extend. The refactor enables per-module testing and clearer ownership of runtime-specific logic.

Changes

• Phase 1 — Test baseline: Added converter, shared utility, and install flow tests (1680+ test lines); added Stryker mutation testing config (69.51% kill rate)
• Phase 2 — Module extraction: Extracted bin/lib/core.js (shared utilities/constants), bin/lib/codex.js, bin/lib/opencode.js, bin/lib/gemini.js, bin/lib/claude.js; migrated test imports from GSD_TEST_MODE to per-module imports
• Phase 3 — Verification: Confirmed 705 tests pass, 28.07% install coverage maintained, backward compat preserved
• Phase 4 — Cleanup: Removed 13 dead imports from install.js, realigned Stryker mutate targets to new modules

Testing

• Tested on macOS
• Tested on Windows
• Tested on Linux

Checklist

• Follows GSD style (no enterprise patterns, no filler)
• Updates CHANGELOG.md for user-facing changes
• No unnecessary dependencies added (Stryker is devDependency only)
• Works on Windows (backslash paths tested)

Breaking Changes

None — all changes are internal. Existing CLI interface, flags, prompts, and output are unchanged. tests/codex-config.test.cjs backward compatibility preserved.