Improve staleness check error handling and optimization (bd-n4td, bd-o4qy, bd-c4rq)

This commit implements three related improvements to database staleness checking:

**bd-n4td (P2): Add warning when staleness check errors**
- Added stderr warnings when CheckStaleness fails in ensureDatabaseFresh
- Users now see "Warning: could not check database staleness: <error>"
- Provides visibility into staleness check failures while allowing operations to proceed

**bd-o4qy (P2): Improve CheckStaleness error handling**
- Updated CheckStaleness to distinguish between expected and abnormal conditions:
  * Returns (false, nil) for expected "no data yet" scenarios (missing metadata, missing JSONL)
  * Returns (false, err) for abnormal errors (glob failures, permission errors)
- Updated RPC server (2 locations) to log staleness errors but allow requests to proceed
- Prevents blocking operations due to transient staleness check issues
- Added comprehensive function documentation

**bd-c4rq (P3): Refactor staleness check to avoid function call overhead**
- Moved daemon check from inside ensureDatabaseFresh to all 8 call sites
- Avoids unnecessary function call when running in daemon mode
- Updated: list.go, info.go, status.go, show.go, stale.go, duplicates.go, ready.go, validate.go
- Extracted staleness functions to new staleness.go for better organization

**Code review fixes:**
- Removed dead code in CheckStaleness (unreachable jsonlPath == "" check)
- Removed unused ensureDatabaseFreshQuiet function

**Files changed:**
- New: cmd/bd/staleness.go (extracted staleness checking functions)
- Modified: 8 command files (added daemon check before staleness calls)
- Modified: internal/autoimport/autoimport.go (improved error handling)
- Modified: internal/rpc/server_export_import_auto.go (handle errors gracefully)

internal/rpc/server_export_import_auto.go

@@ -184,8 +184,13 @@ func (s *Server) checkAndAutoImportIfStale(req *Request) error {
 	// Fast path: Check if JSONL is stale using cheap mtime check
 	// This avoids reading/hashing JSONL on every request
 	isStale, err := autoimport.CheckStaleness(ctx, store, dbPath)
-	if err != nil || !isStale {
-		return err
+	if err != nil {
+		// Log error but allow request to proceed (don't block on staleness check failure)
+		fmt.Fprintf(os.Stderr, "Warning: failed to check staleness: %v\n", err)
+		return nil
+	}
+	if !isStale {
+		return nil
 	}
 
 	// Single-flight guard: Only allow one import at a time
@@ -220,8 +225,13 @@ func (s *Server) checkAndAutoImportIfStale(req *Request) error {
 
 	// Double-check staleness after acquiring lock (another goroutine may have imported)
 	isStale, err = autoimport.CheckStaleness(ctx, store, dbPath)
-	if err != nil || !isStale {
-		return err
+	if err != nil {
+		// Log error but allow request to proceed (don't block on staleness check failure)
+		fmt.Fprintf(os.Stderr, "Warning: failed to check staleness: %v\n", err)
+		return nil
+	}
+	if !isStale {
+		return nil
 	}
 
 	// Create timeout context for import operation (bd-8931, bd-1048)