docs: align MCP auth and client setup with canonical contract

[replynodes-ai]

Summary

Implements replynodes-mcp#25 for parent replynodes-fetcher#433.

• Canonical remote MCP endpoint: https://mcp.replynodes.com/mcp
• Interactive clients: native Better Auth MCP OAuth at https://auth.replynodes.com, scope mcp:read
• Headless/manual and stdio clients: rn_test_* / rn_live_* API keys
• Organization authority comes from the authenticated identity/key; no client-supplied organization id
• Read-only contract and truthful stdio bridge guidance
• Reconciles README, server metadata, Smithery config, Cursor/Claude setup metadata, and CLI messaging
• No Registry, npm, Smithery, or other external listing publication performed

Verification

• node test-cli-secret-safe.mjs
• node --check bin/cli.js
• JSON parse and package/lock/server version consistency checks
• npm pack --dry-run
• Official MCP server.json schema validation
• Repository-local secret scan, prohibited-contract scan, and git diff --check

Parent: replynodes-fetcher#433

[replynodes-ai]

Independent exact-head review APPROVE at b7021fb. Verified canonical endpoint/OAuth/API-key contract, stdio secret safety, no prohibited active claims, JSON/schema/path consistency, node syntax, test-cli-secret-safe, npm pack, and diff-check. Smithery live stale metadata is explicitly documented as external owner-gated.