feat: extract error handling helpers FatalError/WarnError (bd-s0z)

Create reusable error handling helpers to reduce boilerplate and
enforce consistency across the codebase:

- FatalError(format, args...): writes "Error: ..." to stderr and exits
- FatalErrorWithHint(message, hint): includes actionable suggestion
- WarnError(format, args...): writes "Warning: ..." to stderr

Prototyped in create.go to validate the approach - converted 13 error
patterns and 5 warning patterns. This reduces code from:

    fmt.Fprintf(os.Stderr, "Error: %v\n", err)
    os.Exit(1)

To simply:

    FatalError("%v", err)

Also fixed countIssuesInJSONLFile reference after earlier refactoring.

See docs/ERROR_HANDLING.md for the three-pattern guideline.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

cmd/bd/errors.go

@@ -0,0 +1,53 @@
+package main
+
+import (
+	"fmt"
+	"os"
+)
+
+// FatalError writes an error message to stderr and exits with code 1.
+// Use this for fatal errors that prevent the command from completing.
+//
+// Pattern A from ERROR_HANDLING.md:
+// - User input validation failures
+// - Critical preconditions not met
+// - Unrecoverable system errors
+//
+// Example:
+//
+//	if err := store.CreateIssue(ctx, issue, actor); err != nil {
+//	    FatalError("%v", err)
+//	}
+func FatalError(format string, args ...interface{}) {
+	fmt.Fprintf(os.Stderr, "Error: "+format+"\n", args...)
+	os.Exit(1)
+}
+
+// FatalErrorWithHint writes an error message with a hint to stderr and exits.
+// Use this when you can provide an actionable suggestion to fix the error.
+//
+// Example:
+//
+//	FatalErrorWithHint("database not found", "Run 'bd init' to create a database")
+func FatalErrorWithHint(message, hint string) {
+	fmt.Fprintf(os.Stderr, "Error: %s\n", message)
+	fmt.Fprintf(os.Stderr, "Hint: %s\n", hint)
+	os.Exit(1)
+}
+
+// WarnError writes a warning message to stderr and returns.
+// Use this for optional operations that enhance functionality but aren't required.
+//
+// Pattern B from ERROR_HANDLING.md:
+// - Metadata operations
+// - Cleanup operations
+// - Auxiliary features (git hooks, merge drivers)
+//
+// Example:
+//
+//	if err := createConfigYaml(beadsDir, false); err != nil {
+//	    WarnError("failed to create config.yaml: %v", err)
+//	}
+func WarnError(format string, args ...interface{}) {
+	fmt.Fprintf(os.Stderr, "Warning: "+format+"\n", args...)
+}