Remove Gemini engine and all references from codebase

[Copilot]

This PR completely removes the Gemini agentic engine from the gh-aw codebase as requested in the issue. The Gemini engine was one of three supported AI engines (alongside Claude and Codex) but is no longer needed.

Changes Made

Core Engine Removal

• Deleted pkg/workflow/gemini_engine.go - Complete Gemini engine implementation
• Deleted pkg/workflow/gemini_engine_test.go - All Gemini engine tests
• Updated pkg/workflow/agentic_engine.go - Removed Gemini engine registration from NewEngineRegistry()

Test Updates

• Updated pkg/workflow/agentic_engine_test.go - Fixed tests to expect 2 engines instead of 3
• Updated pkg/workflow/engine_config_test.go - Removed Gemini from test configurations
• All existing tests now pass with only Claude and Codex engines

CLI and Validation

• Updated cmd/gh-aw/main.go - Removed "gemini" from CLI validation and help text
• Updated pkg/parser/schemas/main_workflow_schema.json - Removed "gemini" from allowed engine enum values
• CLI now correctly rejects --engine gemini with error: "Must be 'claude' or 'codex'"

Documentation Cleanup

• Updated README.md - Removed Gemini from engine examples
• Updated docs/frontmatter.md - Removed Gemini engine documentation and examples
• Updated docs/secrets.md - Removed GEMINI_API_KEY documentation section
• Updated docs/tools.md - Removed Gemini engine tools section
• Updated pkg/cli/templates/instructions.md - Removed Gemini references from templates

Test Workflows

• Deleted .github/workflows/test-gemini.md - Gemini test workflow markdown
• Deleted .github/workflows/test-gemini.lock.yml - Compiled Gemini test workflow

Validation

The removal has been thoroughly tested:

• ✅ All existing tests pass
• ✅ Build, lint, and compilation successful
• ✅ CLI correctly rejects "gemini" as invalid engine
• ✅ Schema validation prevents "gemini" in workflow frontmatter
• ✅ No remaining "gemini" references found in codebase
• ✅ Claude and Codex engines continue to work normally

The system now cleanly supports only Claude (default) and Codex (experimental) engines, with all Gemini functionality completely removed.

Warning

Firewall rules blocked me from connecting to one or more addresses

I tried to connect to the following addresses, but was blocked by firewall rules:

• api.contoso.com
  • Triggering command: /tmp/go-build791013428/b256/cli.test -test.testlogfile=/tmp/go-build791013428/b256/testlog.txt -test.paniconexit0 -test.timeout=10m0s -test.v=true (dns block)
  • Triggering command: /tmp/go-build3749983207/b256/cli.test -test.testlogfile=/tmp/go-build3749983207/b256/testlog.txt -test.paniconexit0 -test.timeout=10m0s -test.v=true (dns block)
  • Triggering command: /tmp/go-build1284847619/b256/cli.test -test.testlogfile=/tmp/go-build1284847619/b256/testlog.txt -test.paniconexit0 -test.timeout=10m0s -test.v=true (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

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