test: improve internal/compact coverage from 17% to 82% (bd-thgk)

Add comprehensive unit tests for the compact package:

- haiku.go: Mock API tests for SummarizeTier1, retry logic tests for
  callWithRetry (429/500 handling, exhaust retries, context timeout),
  expanded isRetryable tests for network timeouts and API error codes
- git.go: Tests for GetCurrentCommitHash in various git states
  (in repo, outside repo, new repo, empty repo)
- compactor.go: Unit tests for New(), CompactTier1(), CompactTier1Batch()
  with mock API server, config validation, error paths

Small production change: NewHaikuClient now accepts variadic options
for testing (option.WithBaseURL, option.WithMaxRetries).

Coverage: 17.3% → 81.8%

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

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

internal/compact/haiku.go

@@ -38,7 +38,7 @@ type HaikuClient struct {
 }
 
 // NewHaikuClient creates a new Haiku API client. Env var ANTHROPIC_API_KEY takes precedence over explicit apiKey.
-func NewHaikuClient(apiKey string) (*HaikuClient, error) {
+func NewHaikuClient(apiKey string, opts ...option.RequestOption) (*HaikuClient, error) {
 	envKey := os.Getenv("ANTHROPIC_API_KEY")
 	if envKey != "" {
 		apiKey = envKey
@@ -47,7 +47,10 @@ func NewHaikuClient(apiKey string) (*HaikuClient, error) {
 		return nil, fmt.Errorf("%w: set ANTHROPIC_API_KEY environment variable or provide via config", ErrAPIKeyRequired)
 	}
 
-	client := anthropic.NewClient(option.WithAPIKey(apiKey))
+	// Build options: API key first, then any additional options (for testing)
+	allOpts := []option.RequestOption{option.WithAPIKey(apiKey)}
+	allOpts = append(allOpts, opts...)
+	client := anthropic.NewClient(allOpts...)
 
 	tier1Tmpl, err := template.New("tier1").Parse(tier1PromptTemplate)
 	if err != nil {