Add Claude Haiku client for issue compaction (bd-255)

- Implement HaikuClient with Tier 1 and Tier 2 summarization prompts
- Add exponential backoff retry logic for transient errors (network, 429, 5xx)
- API key precedence: env var overrides explicit key parameter
- Comprehensive tests with UTF-8 support and edge case coverage
- Retry logic only retries transient errors, stops on permanent failures
- Clarified retry semantics: maxRetries=3 means 4 total attempts (1 initial + 3 retries)

go.mod

@@ -9,6 +9,7 @@ require (
 )
 
 require (
+	github.com/anthropics/anthropic-sdk-go v1.14.0 // indirect
 	github.com/dustin/go-humanize v1.0.1 // indirect
 	github.com/google/uuid v1.6.0 // indirect
 	github.com/inconshreveable/mousetrap v1.1.0 // indirect
@@ -17,6 +18,10 @@ require (
 	github.com/ncruces/go-strftime v0.1.9 // indirect
 	github.com/remyoudompheng/bigfft v0.0.0-20230129092748-24d4a6f8daec // indirect
 	github.com/spf13/pflag v1.0.10 // indirect
+	github.com/tidwall/gjson v1.18.0 // indirect
+	github.com/tidwall/match v1.1.1 // indirect
+	github.com/tidwall/pretty v1.2.1 // indirect
+	github.com/tidwall/sjson v1.2.5 // indirect
 	golang.org/x/exp v0.0.0-20250620022241-b7579e27df2b // indirect
 	golang.org/x/sys v0.34.0 // indirect
 	modernc.org/libc v1.66.3 // indirect