Remove unreachable utility functions (bd-224, bd-214)

- Remove duplicate computeIssueContentHash from sqlite/hash.go
- Remove FileUsed() from internal/config/config.go
- Remove verifyIssueOpen() test helper from git_sync_test.go
- Remove unimplemented SummarizeTier2 and all tier2 infrastructure from haiku.go

Removes ~120 LOC of dead code identified by deadcode analyzer.

Amp-Thread-ID: https://ampcode.com/threads/T-5f150c35-8d67-4dae-bb92-a7b5887d649d
Co-authored-by: Amp <amp@ampcode.com>

internal/config/config.go

@@ -147,14 +147,6 @@ func Set(key string, value interface{}) {
 // 	return v.BindPFlag(key, flag)
 // }
 
-// FileUsed returns the path to the active configuration file.
-func FileUsed() string {
-	if v == nil {
-		return ""
-	}
-	return v.ConfigFileUsed()
-}
-
 // AllSettings returns all configuration settings as a map
 func AllSettings() map[string]interface{} {
 	if v == nil {