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>

internal/rpc/rpc_test.go

@@ -38,6 +38,14 @@ func setupTestServer(t *testing.T) (*Server, *Client, func()) {
 		t.Fatalf("Failed to create store: %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()
+		os.RemoveAll(tmpDir)
+		t.Fatalf("Failed to set issue_prefix: %v", err)
+	}
+
 	server := NewServer(socketPath, store, tmpDir, dbPath)
 
 	ctx, cancel := context.WithCancel(context.Background())