fix(cli): remove antigravity auto-injection from install

[jsl9208]

Summary

• Remove automatic antigravity plugin and provider configuration injection from install process
• Users upgrading to v3 were experiencing their existing antigravity configs being overwritten
• Installation docs already guide LLM/users through manual configuration at the OpenCode level

Changes

• config-manager.ts: Remove ANTIGRAVITY_PROVIDER_CONFIG constant, addProviderConfig(), and addAuthPlugins() functions
• install.ts: Remove injection calls, update totalSteps from 6 to 4, change Gemini hint to "Install opencode-antigravity-auth first"
• config-manager.test.ts: Remove related test block

Background: While upgrading to v3, my existing opencode-antigravity-auth provider/model configs were overwritten by hardcoded values. Found PR #812 was closed with maintainer comment: "Antigravity is no longer supported directly in oMo. It will still work but config will happen in OpenCode itself."

This PR takes the complete approach by removing auto-injection entirely. The installation guide already provides complete instructions for manual setup.

Testing

bun run typecheck  # pass
bun test src/cli/  # 181 pass, 0 fail

Manual verification:

# Backup existing config
cp ~/.config/opencode/opencode.json ~/.config/opencode/opencode.json.bak

# Run install with --gemini=yes
bun ./src/cli/index.ts install --no-tui --claude=yes --gemini=yes --copilot=no --skip-auth

# Verify provider config unchanged
diff ~/.config/opencode/opencode.json.bak ~/.config/opencode/opencode.json
# (no output = identical)

Related Issues

Related to #812

---

Summary by cubic

Removed automatic Antigravity plugin/provider injection from CLI install to stop overwriting existing user configs. Install now leaves Antigravity setup to manual configuration in OpenCode and reduces steps from 6 to 4.

• Refactors

  • Deleted ANTIGRAVITY_PROVIDER_CONFIG, addProviderConfig, and addAuthPlugins.
  • Removed related calls in install flow; updated step count to 4.
  • Updated Gemini auth hint to “Install opencode-antigravity-auth first.”
  • Removed associated test block.

• Migration

  • Antigravity is no longer auto-configured during install.
  • Install opencode-antigravity-auth manually and configure in OpenCode per docs.
  • Existing Antigravity settings are preserved during upgrades.

^{Written for commit 5d9ab3b. Summary will update on new commits.}

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[jsl9208]

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[jsl9208]

recheck

[cubic-dev-ai]

No issues found across 3 files

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.