feat(hyperguild): brain write subcommand

Reads markdown from stdin, POSTs to the brain's /write endpoint with
type + slug, prints the resulting path. Pairs with 'brain query' for
shell-friendly read/write access to the brain HTTP REST API.

Tests cover success, missing args, backend error propagation, and
empty stdin (which produces an empty content payload — the brain
server's responsibility to validate).

cmd/hyperguild/brain.go

@@ -52,8 +52,22 @@ func runBrainQuery(ctx context.Context, args []string, _ io.Reader, stdout, stde
 	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)")
+func runBrainWrite(ctx context.Context, args []string, stdin io.Reader, stdout, stderr io.Writer) error {
+	fs := flag.NewFlagSet("brain write", flag.ContinueOnError)
+	fs.SetOutput(stderr)
+	if err := fs.Parse(args); err != nil {
+		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)")
+	}
+	kind := fs.Arg(0)
+	slug := fs.Arg(1)
+
+	res, err := newBrainClient().Write(ctx, kind, slug, stdin)
+	if err != nil {
+		return err
+	}
+	fmt.Fprintln(stdout, res.Path) //nolint:errcheck
+	return nil
 }