fix: correct stale twitter flag, MP3 case match, reddit tier, bird residue (#344)

Four small correctness fixes found while live-testing community PRs:

- SKILL.md taught agents 'twitter search --limit 10' but twitter-cli
  v0.8.5 has no --limit (real flag is -n/--max) — every agent following
  the skill got a usage error
- transcribe_xiaoyuzhou.sh audio regex only matched lowercase .m4a/.mp3;
  current episodes serve uppercase .MP3 (verified on a live episode) —
  add /i flag
- reddit.py declared tier=0 (zero-config) while its own docstring says
  Reddit requires login since 2024 — set tier=1 to match reality (tier
  is doctor display metadata only)
- docs/README_en.md referenced the retired 'bird' CLI name once

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

agent_reach/channels/reddit.py

@@ -19,7 +19,7 @@ class RedditChannel(Channel):
     name = "reddit"
     description = "Reddit 帖子和评论"
     backends = ["rdt-cli"]
-    tier = 0
+    tier = 1  # Reddit requires login since 2024 (rdt login) — not zero-config
 
     def can_handle(self, url: str) -> bool:
         from urllib.parse import urlparse