fix(twitter): support both twitter-cli and bird CLI, never uninstall user tools

CRITICAL FIX: update.md was telling agents to uninstall bird CLI,
which broke users who had working bird installations. Now:

- twitter.py: prefers twitter-cli, falls back to bird/birdx if installed
- update.md: removed "clean up deprecated tools" step entirely
- Added explicit rule: "Never uninstall any existing tools the user already has"
- Tests cover twitter-cli primary + bird fallback + preference order

78 tests passing.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/update.md

@@ -71,15 +71,9 @@ which yt-dlp >/dev/null 2>&1 || pip install yt-dlp 2>/dev/null
 which gh >/dev/null 2>&1 || echo "GitHub CLI not installed — see https://cli.github.com"
 ```
 
-### Step 4: Clean up deprecated tools
+### Step 4: Coexistence (DO NOT uninstall old tools)
 
-```bash
-# Remove bird CLI (repo deleted, replaced by twitter-cli)
-which bird >/dev/null 2>&1 && npm uninstall -g @steipete/bird 2>/dev/null && echo "✅ Removed deprecated bird CLI"
-
-# Note: browser_cookie3 is still supported as fallback, no need to remove
-# Note: miku_ai is no longer needed (WeChat now uses Exa), but no harm keeping it
-```
+**IMPORTANT: Never uninstall any existing tools the user already has installed.** Old tools (bird, browser_cookie3, miku_ai, etc.) may still be working on the user's machine even if their upstream repos are archived or deleted. Agent Reach supports both old and new tools as fallback. Only install new tools alongside existing ones.
 
 ### Step 5: Verify