feat(mcp): add load_diagram tool to load .drawio files into the session (#893)

* feat(mcp): add load_diagram tool to load .drawio files into the session

Loading a file previously required the agent to read the file itself and
pass the entire XML through create_new_diagram - wasteful for large
diagrams and impossible for draw.io's compressed save format.

load_diagram takes a file path; the server reads it, decompresses any
compressed pages (base64 -> raw deflate -> URI-decode, per page), and
replaces the session document. The loaded XML is deliberately NOT marked
as seen by the edit gate: the model only supplied a path, so it must
call get_diagram once before editing.

* chore(mcp): version 0.2.3

* fix(mcp): report package.json version in the MCP handshake

The McpServer metadata version was a separate hardcoded string that
never matched the published version (stuck at 0.1.2, then 0.3.0 while
npm shipped 0.2.x). Read it from package.json at startup instead —
works from both src/ (tsx) and dist/ (published build).

packages/mcp-server/tests/server-wiring.test.ts

@@ -31,6 +31,7 @@ const tsxBin = path.resolve(
 const EXPECTED_TOOLS = [
     "start_session",
     "create_new_diagram",
+    "load_diagram",
     "edit_diagram",
     "get_diagram",
     "export_diagram",