feat(mcp): ready for a remote server: quiet logs and allowed domains

- Logs default to warnings and errors (MCP_LOG_LEVEL): the library logged
  every tool on every request (~48 lines per call).
- DNS rebinding protection on /mcp, as the MCP spec asks of HTTP servers:
  the Host header must be in MCP_ALLOWED_HOSTS (default: localhost; compose
  default mcp.taskstream.pireon.dev,localhost) and a browser Origin, when
  present, too; otherwise 403 with a JSON-RPC error. Uses the official
  validators from @modelcontextprotocol/server.
- Test for allowed/foreign Host and Origin; .env.example, compose and
  docs/09-mcp.md document the variables; Word/PDF regenerated.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.env.example

@@ -39,3 +39,11 @@ OPENAPI_FILE=apps/api/openapi.json
 # Docker builds only: token of Pireon's Nexus for @pi/ui (build secret, never stored in an image).
 # Locally: export NEXUS_TOKEN=$(grep '^//nexus.pireon.dev/repository/npm-releases/:_authToken=' ~/.npmrc | cut -d= -f2-)
 NEXUS_TOKEN=
+
+# MCP server (apps/mcp, docs/09-mcp.md). Port: not PORT, which is the API's.
+MCP_PORT=3010
+TASKSTREAM_API_URL=http://localhost:3000
+# Domains it answers on, comma-separated (DNS rebinding protection). Empty: localhost only.
+MCP_ALLOWED_HOSTS=
+# error | warn | log | debug | verbose (warn: the library logs every tool on every call otherwise)
+MCP_LOG_LEVEL=warn