feat(agent): add Gemini CLI install target

[merlinsantiago982-cmd]

Summary

• add a gemini agent target that installs TestSprite guidance into GEMINI.md
• generalize managed-section metadata so Codex and Gemini can each preserve user-authored root instruction files
• update agent docs, help snapshots, and install/list coverage

Safety

• GEMINI.md uses sentinel-delimited managed-section writes, so existing project instructions outside the TestSprite block are preserved
• re-running install replaces only the TestSprite section

Tests

• npm run format:check
• npm run test -- src/lib/agent-targets.test.ts src/commands/agent.test.ts test/help.snapshot.test.ts
• npm run typecheck
• npm run lint
• npm run build