fix(reddit): switch to Exa-only for search and read, remove proxy dependency

Reddit blocks nearly all non-browser access at IP level (including ISP proxies).
Switch Reddit channel to use Exa exclusively:
- Search: web_search_exa with includeDomains: ["reddit.com"]
- Read: crawling_exa for full post + comments
- check() now verifies Exa availability instead of probing Reddit API
- tier changed from 1 (needs config) to 0 (zero config)
- Removed reddit_proxy from config, CLI, and setup guide
- Updated all docs (README, README_en, install.md, SKILL.md, references)
- Fixed xreach→bird references in references/social.md

Fixes #218, Fixes #222, Fixes #221

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

agent_reach/config.py

@@ -21,7 +21,6 @@ class Config:
     # Feature → required config keys
     FEATURE_REQUIREMENTS = {
         "exa_search": ["exa_api_key"],
-        "reddit_proxy": ["reddit_proxy"],
         "twitter_xreach": ["twitter_auth_token", "twitter_ct0"],  # legacy key name; used by bird CLI
         "groq_whisper": ["groq_api_key"],
         "github_token": ["github_token"],