Add imessage channel plugin

iMessage bridge for Claude Code. Reads ~/Library/Messages/chat.db
directly for history and new-message polling; sends via AppleScript
to Messages.app. macOS only.

Built-in access control: inbound messages are gated by an allowlist
(default: self-chat only), outbound sends are scoped to the same
allowlist. The /imessage:access skill manages allowlists and policy.

Requires Full Disk Access and Automation TCC grants — both prompted
by macOS on first use.

Ships full source — server.ts runs locally via bun, started by the
.mcp.json command.

external_plugins/imessage/.mcp.json

@@ -0,0 +1,8 @@
+{
+  "mcpServers": {
+    "imessage": {
+      "command": "bun",
+      "args": ["run", "--cwd", "${CLAUDE_PLUGIN_ROOT}", "--shell=bun", "--silent", "start"]
+    }
+  }
+}