feat(mcp): Model Context Protocol host extension

New `mcp` standard extension (`packages/mcp/`) that makes Dispatch an MCP
host: spawns configured MCP servers (stdio child processes), performs the MCP
handshake (initialize → notifications/initialized), discovers tools via
tools/list, and registers each as a first-class Dispatch ToolContract via
host.defineTool. When the model calls an MCP tool, the extension proxies the
call to tools/call on the MCP server and returns the flattened result.

Architecture (sibling of `lsp` extension):
- Config: .dispatch/mcp.json (servers key) → opencode.json mcp key fallback,
  resolved per-cwd (mirrors LSP config resolution)
- Transport: StdioTransport (spawn child, Content-Length framing + JSON-RPC 2.0)
- Client: initialize → tools/list → tools/call; handles list_changed
  notifications for dynamic tool updates
- Registry: tool name namespacing (<serverId>__<toolName>), ToolContract
  adapter that proxies execute → callTool, content flattening (text/image/
  resource → string)
- Manager: one client per server, lazy-spawn, status(), shutdownAll()
- Extension: manifest (dependsOn session-orchestrator, capabilities spawn),
  registers tools + a toolsFilter (drops disconnected server's tools),
  mcpServiceHandle, deactivate kills all child processes

Phase 1 scope: stdio only, Tools only (no Resources/Prompts/HTTP/sampling).
Hand-rolled JSON-RPC + framing (zero external deps, adapts LSP patterns).

Wave 1 (agent): 12 source + 8 test files, 69 new tests.
Wave 2 (orchestrator): root tsconfig ref, host-bin CORE_EXTENSIONS
registration + package.json dep, bun install.

Verified: tsc -b EXIT 0, biome clean, 1537 vitest pass (was 1468, +69).

1 files changed, 108 insertions, 35 deletions

diff --git a/tsconfig.json b/tsconfig.json
index 7babef3..3dea4a1 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -1,40 +1,113 @@
 {
 	"files": [],
 	"references": [
-		{ "path": "./packages/wire" },
-		{ "path": "./packages/kernel" },
-		{ "path": "./packages/transport-contract" },
-		{ "path": "./packages/ui-contract" },
-		{ "path": "./packages/surface-registry" },
-		{ "path": "./packages/transport-ws" },
-		{ "path": "./packages/surface-loaded-extensions" },
-		{ "path": "./packages/storage-sqlite" },
-		{ "path": "./packages/auth-apikey" },
-		{ "path": "./packages/provider-openai-compat" },
-		{ "path": "./packages/openai-stream" },
-		{ "path": "./packages/provider-umans" },
-		{ "path": "./packages/credential-store" },
-		{ "path": "./packages/conversation-store" },
-		{ "path": "./packages/throughput-store" },
-		{ "path": "./packages/todo" },
-		{ "path": "./packages/session-orchestrator" },
-		{ "path": "./packages/transport-http" },
-		{ "path": "./packages/tool-read-file" },
-		{ "path": "./packages/tool-shell" },
-		{ "path": "./packages/tool-edit-file" },
-		{ "path": "./packages/tool-write-file" },
-		{ "path": "./packages/tool-web-search" },
-		{ "path": "./packages/tool-youtube-transcript" },
-		{ "path": "./packages/skills" },
-		{ "path": "./packages/cache-warming" },
-		{ "path": "./packages/message-queue" },
-		{ "path": "./packages/lsp" },
-		{ "path": "./packages/system-prompt" },
-		{ "path": "./packages/cli" },
-		{ "path": "./packages/journal-sink" },
-		{ "path": "./packages/trace-store" },
-		{ "path": "./packages/observability-collector" },
-		{ "path": "./packages/trace-replay" },
-		{ "path": "./packages/host-bin" }
+		{
+			"path": "./packages/wire"
+		},
+		{
+			"path": "./packages/kernel"
+		},
+		{
+			"path": "./packages/transport-contract"
+		},
+		{
+			"path": "./packages/ui-contract"
+		},
+		{
+			"path": "./packages/surface-registry"
+		},
+		{
+			"path": "./packages/transport-ws"
+		},
+		{
+			"path": "./packages/surface-loaded-extensions"
+		},
+		{
+			"path": "./packages/storage-sqlite"
+		},
+		{
+			"path": "./packages/auth-apikey"
+		},
+		{
+			"path": "./packages/provider-openai-compat"
+		},
+		{
+			"path": "./packages/openai-stream"
+		},
+		{
+			"path": "./packages/provider-umans"
+		},
+		{
+			"path": "./packages/credential-store"
+		},
+		{
+			"path": "./packages/conversation-store"
+		},
+		{
+			"path": "./packages/throughput-store"
+		},
+		{
+			"path": "./packages/todo"
+		},
+		{
+			"path": "./packages/session-orchestrator"
+		},
+		{
+			"path": "./packages/transport-http"
+		},
+		{
+			"path": "./packages/tool-read-file"
+		},
+		{
+			"path": "./packages/tool-shell"
+		},
+		{
+			"path": "./packages/tool-edit-file"
+		},
+		{
+			"path": "./packages/tool-write-file"
+		},
+		{
+			"path": "./packages/tool-web-search"
+		},
+		{
+			"path": "./packages/tool-youtube-transcript"
+		},
+		{
+			"path": "./packages/skills"
+		},
+		{
+			"path": "./packages/cache-warming"
+		},
+		{
+			"path": "./packages/message-queue"
+		},
+		{
+			"path": "./packages/mcp"
+		},
+		{
+			"path": "./packages/lsp"
+		},
+		{
+			"path": "./packages/system-prompt"
+		},
+		{
+			"path": "./packages/cli"
+		},
+		{
+			"path": "./packages/journal-sink"
+		},
+		{
+			"path": "./packages/trace-store"
+		},
+		{
+			"path": "./packages/observability-collector"
+		},
+		{
+			"path": "./packages/trace-replay"
+		},
+		{
+			"path": "./packages/host-bin"
+		}
 	]
 }