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>

cmd/bd/git_sync_test.go

@@ -296,13 +296,3 @@ func verifyIssueClosed(t *testing.T, store *sqlite.SQLiteStorage, issueID string
 		t.Errorf("Expected issue %s to be closed, got status %s", issueID, issue.Status)
 	}
 }
-
-func verifyIssueOpen(t *testing.T, store *sqlite.SQLiteStorage, issueID string) {
-	issue, err := store.GetIssue(context.Background(), issueID)
-	if err != nil {
-		t.Fatalf("Failed to get issue %s: %v", issueID, err)
-	}
-	if issue.Status != types.StatusOpen {
-		t.Errorf("Expected issue %s to be open, got status %s", issueID, issue.Status)
-	}
-}