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, 11 insertions, 0 deletions

diff --git a/bun.lock b/bun.lock
index 44e7bcd..3671dee 100644
--- a/bun.lock
+++ b/bun.lock
@@ -62,6 +62,7 @@
         "@dispatch/journal-sink": "workspace:*",
         "@dispatch/kernel": "workspace:*",
         "@dispatch/lsp": "workspace:*",
+        "@dispatch/mcp": "workspace:*",
         "@dispatch/message-queue": "workspace:*",
         "@dispatch/provider-openai-compat": "workspace:*",
         "@dispatch/provider-umans": "workspace:*",
@@ -104,6 +105,14 @@
         "@dispatch/kernel": "workspace:*",
       },
     },
+    "packages/mcp": {
+      "name": "@dispatch/mcp",
+      "version": "0.0.0",
+      "dependencies": {
+        "@dispatch/kernel": "workspace:*",
+        "@dispatch/session-orchestrator": "workspace:*",
+      },
+    },
     "packages/message-queue": {
       "name": "@dispatch/message-queue",
       "version": "0.0.0",
@@ -349,6 +358,8 @@
 
     "@dispatch/lsp": ["@dispatch/lsp@workspace:packages/lsp"],
 
+    "@dispatch/mcp": ["@dispatch/mcp@workspace:packages/mcp"],
+
     "@dispatch/message-queue": ["@dispatch/message-queue@workspace:packages/message-queue"],
 
     "@dispatch/observability-collector": ["@dispatch/observability-collector@workspace:packages/observability-collector"],