feat(mcp): streamable HTTP transport with session, init, and dispatch

Implements the Streamable HTTP transport: POST routing handles initialize
(issues session ID), tools/list, tools/call, and unknown methods; GET SSE
emits a keepalive comment then blocks on context cancellation. A minimal
registry stub is introduced so the server compiles and tools/list returns
an empty array until Phase 6+ registers real tools.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/registry/registry.go

@@ -0,0 +1,42 @@
+package registry
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+)
+
+type ToolDescriptor struct {
+	Name        string          `json:"name"`
+	Description string          `json:"description"`
+	InputSchema json.RawMessage `json:"inputSchema"`
+}
+
+type Tool interface {
+	Descriptor() ToolDescriptor
+	Call(ctx context.Context, args json.RawMessage) (json.RawMessage, error)
+}
+
+type Registry struct {
+	tools map[string]Tool
+}
+
+func New() *Registry { return &Registry{tools: map[string]Tool{}} }
+
+func (r *Registry) Register(t Tool) { r.tools[t.Descriptor().Name] = t }
+
+func (r *Registry) Tools() []ToolDescriptor {
+	out := make([]ToolDescriptor, 0, len(r.tools))
+	for _, t := range r.tools {
+		out = append(out, t.Descriptor())
+	}
+	return out
+}
+
+func (r *Registry) Dispatch(ctx context.Context, name string, args json.RawMessage) (json.RawMessage, error) {
+	t, ok := r.tools[name]
+	if !ok {
+		return nil, errors.New("tool not found: " + name)
+	}
+	return t.Call(ctx, args)
+}