docs: backfill rednote MCP usage documentation

[yimin12]

Summary

• Backfill README for the current rednote-mcp repository scope.
• Document actual package scripts, dist/index.js entry point, setup, Claude Desktop configuration, login/cookie behavior, generated directories, troubleshooting, and limitations.
• Document both MCP tools, search_xiaohongshu and list_favorites, including parameters and output.
• Add the list_favorites MCP registration and Playwright favorites scraper so the documented tool exists in the PR implementation.
• Tighten favorites profile navigation so it only uses visible account/navigation profile links and does not fall back to feed author profile links.
• Correct stale references to build/index.js, pnpm test, pnpm clean, and describe the current LICENSE/package.json license metadata mismatch without treating package.json as authoritative.

Verification

• pnpm build

Notes

• /Volumes/Portal_SSD/pojo is not a Git repository; this PR updates /Volumes/Portal_SSD/pojo/rednote-mcp only.
• A local untracked node_modules symlink exists in the linked review worktree and is not part of the PR.