feat(mcp): add HTTP transport support and tool namespacing

MCP tools are now prefixed with their server name (e.g., vigil-harbor:memory_status)
to prevent collisions between tools from different MCP servers and built-in tools.

Adds SSE and StreamableHTTP transport support alongside existing stdio, enabling
connection to remote MCP servers via URL-based config with optional custom headers
and env var substitution. Includes config validation, session lifecycle management,
and 5 new tests for HTTP config edge cases.

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

src/agents/pi-embedded-runner.bundle-mcp.e2e.test.ts

@@ -55,7 +55,7 @@ vi.mock("./pi-bundle-mcp-tools.js", () => ({
   materializeBundleMcpToolsForRun: async () => ({
     tools: [
       {
-        name: "bundle_probe",
+        name: "bundleProbe__bundle_probe",
         label: "bundle_probe",
         description: "Bundle MCP probe",
         parameters: { type: "object", properties: {} },
@@ -81,7 +81,7 @@ vi.mock("@mariozechner/pi-ai", async (importOriginal) => {
       {
         type: "toolCall" as const,
         id: "tc-bundle-mcp-1",
-        name: "bundle_probe",
+        name: "bundleProbe__bundle_probe",
         arguments: {},
       },
     ],