fix(hyperguild): remove redundant subcommand prefixes from error messages

dispatch() already prefixes errors with 'hyperguild <subcmd>: ', so
handlers re-prefixing with their own name produced stuttered output
like 'hyperguild brain: brain query: topic required'. Strip the
redundant prefixes from the seven affected errors.New / fmt.Errorf
calls in brain.go and mode.go.

Also fix the LITELLM_BASE_URL usage text — it's optional (empty
falls through to airplane tier), not required, matching the
README.

cmd/hyperguild/brain.go

@@ -11,7 +11,7 @@ import (
 
 func runBrain(ctx context.Context, args []string, stdin io.Reader, stdout, stderr io.Writer) error {
 	if len(args) == 0 {
-		return errors.New("brain: subcommand required (query|write)")
+		return errors.New("subcommand required (query|write)")
 	}
 	switch args[0] {
 	case "query":
@@ -19,7 +19,7 @@ func runBrain(ctx context.Context, args []string, stdin io.Reader, stdout, stder
 	case "write":
 		return runBrainWrite(ctx, args[1:], stdin, stdout, stderr)
 	default:
-		return fmt.Errorf("brain: unknown subcommand: %s (expected query|write)", args[0])
+		return fmt.Errorf("unknown subcommand: %s (expected query|write)", args[0])
 	}
 }
 
@@ -32,7 +32,7 @@ func runBrainQuery(ctx context.Context, args []string, _ io.Reader, stdout, stde
 		return fmt.Errorf("parse flags: %w", err)
 	}
 	if fs.NArg() < 1 {
-		return errors.New("brain query: topic required")
+		return errors.New("topic required")
 	}
 	topic := fs.Arg(0)
 
@@ -59,7 +59,7 @@ func runBrainWrite(ctx context.Context, args []string, stdin io.Reader, stdout,
 		return fmt.Errorf("parse flags: %w", err)
 	}
 	if fs.NArg() < 2 {
-		return errors.New("brain write: type and slug required (e.g. brain write knowledge my-slug)")
+		return errors.New("type and slug required (e.g. brain write knowledge my-slug)")
 	}
 	kind := fs.Arg(0)
 	slug := fs.Arg(1)

cmd/hyperguild/main.go

@@ -39,7 +39,7 @@ Environment:
   ANTHROPIC_PROBE_URL    Tier probe URL for the Anthropic API.
                          Default: https://api.anthropic.com
   LITELLM_BASE_URL       Tier probe URL for the LiteLLM gateway.
-                         Required for tier probe; no default.
+                         Optional; if empty, falls through to airplane tier.
 `
 
 // dispatch routes args to a subcommand and returns the process exit code.

cmd/hyperguild/mode.go

@@ -18,7 +18,7 @@ func runMode(ctx context.Context, args []string, _ io.Reader, stdout, stderr io.
 	// Pull the first positional (mode name) out so flags after it still parse
 	// with stdlib flag (which stops at the first non-flag arg).
 	if len(args) < 1 {
-		return errors.New("mode: name required (cloud|client-local|sovereign)")
+		return errors.New("name required (cloud|client-local|sovereign)")
 	}
 	name := args[0]
 	if err := fs.Parse(args[1:]); err != nil {
@@ -39,12 +39,12 @@ func runMode(ctx context.Context, args []string, _ io.Reader, stdout, stderr io.
 	case "sovereign":
 		doc = modeSovereign(brainURL)
 	default:
-		return fmt.Errorf("mode: unknown mode: %s (expected cloud|client-local|sovereign)", name)
+		return fmt.Errorf("unknown mode: %s (expected cloud|client-local|sovereign)", name)
 	}
 
 	if !*force {
 		if _, err := os.Stat(*out); err == nil {
-			return fmt.Errorf("mode: %s exists (use --force to overwrite)", *out)
+			return fmt.Errorf("%s exists (use --force to overwrite)", *out)
 		}
 	}