Fix bd-166: Prevent duplicate issues with wrong prefix

Critical fix for silent data corruption where database created 173
duplicate issues with wrong prefix (beads- instead of bd-).

Root cause: When issue_prefix config was missing, CreateIssue fell
back to deriving prefix from database filename (beads.db → 'beads'),
while auto-import imported bd- issues from git with SkipPrefixValidation.
This created duplicates.

Changes:
1. Removed derivePrefixFromPath() - never derive prefix from filename
2. CreateIssue/CreateIssues now REJECT if issue_prefix config missing
   - Fail-fast with clear error message
3. Auto-import now SETS issue_prefix from first imported issue if missing
   - Handles fresh clone scenario safely
4. Added newTestStore() helper that sets issue_prefix for tests
5. Updated test setup in multiple files to prevent test failures

Follow-ups filed: bd-167, bd-168, bd-169

Closes bd-166

Amp-Thread-ID: https://ampcode.com/threads/T-b2ee0738-b90b-40ef-ae44-f2d93729842c
Co-authored-by: Amp <amp@ampcode.com>

cmd/bd/test_helpers_test.go

@@ -0,0 +1,28 @@
+package main
+
+import (
+	"context"
+	"testing"
+
+	"github.com/steveyegge/beads/internal/storage/sqlite"
+)
+
+// newTestStore creates a SQLite store with issue_prefix configured (bd-166)
+// This prevents "database not initialized" errors in tests
+func newTestStore(t *testing.T, dbPath string) *sqlite.SQLiteStorage {
+	t.Helper()
+	
+	store, err := sqlite.New(dbPath)
+	if err != nil {
+		t.Fatalf("Failed to create test database: %v", err)
+	}
+	
+	// CRITICAL (bd-166): Set issue_prefix to prevent "database not initialized" errors
+	ctx := context.Background()
+	if err := store.SetConfig(ctx, "issue_prefix", "bd"); err != nil {
+		store.Close()
+		t.Fatalf("Failed to set issue_prefix: %v", err)
+	}
+	
+	return store
+}