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>

tests/test_config.py

@@ -61,11 +61,6 @@ class TestConfig:
         tmp_config.set("exa_api_key", "test-key")
         assert tmp_config.is_configured("exa_search")
 
-    def test_is_configured_reddit(self, tmp_config):
-        assert not tmp_config.is_configured("reddit_proxy")
-        tmp_config.set("reddit_proxy", "http://user:pass@ip:port")
-        assert tmp_config.is_configured("reddit_proxy")
-
     def test_get_configured_features(self, tmp_config):
         features = tmp_config.get_configured_features()
         assert isinstance(features, dict)