v2.1: Bundle Bird X search - no external CLI needed

Vendor Bird's Twitter GraphQL search client directly into /last30days,
eliminating the dependency on `npm install -g @steipete/bird`. X search
now works out of the box with just Node.js 22+ and browser cookies.

- Add vendored bird-search.mjs wrapper (search-only subset of Bird v0.8.0)
- Vendor @steipete/sweet-cookie for browser cookie extraction
- Update bird_x.py to call vendored Node.js module instead of `bird` binary
- Update README.md and SKILL.md for v2.1 (remove Bird CLI install steps)
- Include Bird's MIT LICENSE in vendor directory

The fallback chain is: vendored search -> xAI API key -> web-only mode.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

scripts/lib/vendor/bird-search/package.json

@@ -0,0 +1,13 @@
+{
+  "name": "bird-search",
+  "version": "0.8.0",
+  "description": "Vendored Bird CLI search subset for /last30days",
+  "type": "module",
+  "main": "bird-search.mjs",
+  "private": true,
+  "engines": {
+    "node": ">=22"
+  },
+  "license": "MIT",
+  "attribution": "Based on @steipete/bird v0.8.0 by Peter Steinberger (MIT License)"
+}