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/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.