feat: add bd setup gemini for Gemini CLI integration (#845)

Add support for Gemini CLI hook-based integration, similar to Claude Code:
- bd setup gemini: Install SessionStart/PreCompress hooks
- bd setup gemini --check: Verify installation
- bd setup gemini --remove: Remove hooks
- bd setup gemini --project: Project-only installation
- bd setup gemini --stealth: Use bd prime --stealth

Also adds Gemini CLI integration check to bd doctor.

Gemini CLI's hook system is nearly identical to Claude Code's,
making this a clean, low-maintenance addition.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

cmd/bd/doctor.go

@@ -382,6 +382,11 @@ func runDiagnostics(path string) doctorResult {
 	result.Checks = append(result.Checks, claudeCheck)
 	// Don't fail overall check for missing Claude integration, just warn
 
+	// Check 11b: Gemini CLI integration
+	geminiCheck := convertWithCategory(doctor.CheckGemini(), doctor.CategoryIntegration)
+	result.Checks = append(result.Checks, geminiCheck)
+	// Don't fail overall check for missing Gemini integration, just info
+
 	// Check 11a: bd in PATH (needed for Claude hooks to work)
 	bdPathCheck := convertWithCategory(doctor.CheckBdInPath(), doctor.CategoryIntegration)
 	result.Checks = append(result.Checks, bdPathCheck)