Rollback standalone embedded webhook listener

[lipluscodex]

purpose

Revert the embedded standalone webhook listener introduced in v0.4.0 and return github-webhook-mcp to the previous split model where webhook reception is handled separately from the MCP reader.

premise

• standalone embedded listening only works while the stdio MCP process is alive
• webhook delivery must not depend on the AI client currently running
• v0.4.0 added embedded listener/config/docs that need to be removed

constraints

• keep the Anthropic local directory submission improvements from v0.3.1
• remove standalone listener wiring from the Node MCP server and plugin config surface
• align docs/package metadata with the split receiver model again