feat(hyperguild): brain query subcommand

Adds 'hyperguild brain query <topic>' against the brain HTTP REST
/query endpoint. Default human output prints path + score + title;
--json passes through the response envelope. --limit overrides the
default 5-result cap.

runBrainWrite remains a stub for Task 5.

cmd/hyperguild/brain.go

@@ -0,0 +1,59 @@
+package main
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"flag"
+	"fmt"
+	"io"
+)
+
+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)")
+	}
+	switch args[0] {
+	case "query":
+		return runBrainQuery(ctx, args[1:], stdin, stdout, stderr)
+	case "write":
+		return runBrainWrite(ctx, args[1:], stdin, stdout, stderr)
+	default:
+		return fmt.Errorf("brain: unknown subcommand: %s (expected query|write)", args[0])
+	}
+}
+
+func runBrainQuery(ctx context.Context, args []string, _ io.Reader, stdout, stderr io.Writer) error {
+	fs := flag.NewFlagSet("brain query", flag.ContinueOnError)
+	fs.SetOutput(stderr)
+	asJSON := fs.Bool("json", false, "output JSON instead of human-readable")
+	limit := fs.Int("limit", 5, "maximum number of results")
+	if err := fs.Parse(args); err != nil {
+		return fmt.Errorf("parse flags: %w", err)
+	}
+	if fs.NArg() < 1 {
+		return errors.New("brain query: topic required")
+	}
+	topic := fs.Arg(0)
+
+	res, err := newBrainClient().Query(ctx, topic, *limit)
+	if err != nil {
+		return err
+	}
+
+	if *asJSON {
+		enc := json.NewEncoder(stdout)
+		enc.SetIndent("", "  ")
+		return enc.Encode(res)
+	}
+	for _, hit := range res.Results {
+		fmt.Fprintf(stdout, "%s  score=%d  %s\n", hit.Path, hit.Score, hit.Title) //nolint:errcheck
+	}
+	return nil
+}
+
+// runBrainWrite is implemented in Task 5; stub now returns an explicit error
+// so the router compiles and tests for runBrainQuery can run.
+func runBrainWrite(_ context.Context, _ []string, _ io.Reader, _, _ io.Writer) error {
+	return errors.New("brain write: not implemented (Task 5)")
+}