refactor(adapters): centralize curation in config/queries.json (issue #7)

- adapters/__init__.py: add load_queries() + source_config() (stdlib json,
  safe fallback to {} on missing/corrupt config so pipeline never crashes).
- config/queries.json: per-adapter blocks (hackernews.keywords, arxiv.categories,
  reddit.subreddits, rss.feeds+keywords, github.search_terms). JSON (not
  yaml) to honor Athena's dependency-free runtime; PyYAML avoided.
- hackernews: drop class AI_KEYWORDS + the DUPLICATE inline list inside
  _is_ai_relevant() (the internal drift Ty flagged). Now loads self.ai_keywords
  from config. Simplified matching to single substring pass (boundary variants
  'ai ',' ai','ai-','-ai' approximate word-boundary; dropped the niche
  'compute+tech-context' guard as not worth centralizing).
- reddit: DEFAULT_SUBREDDITS kept as fallback; __init__ prefers config.
- arxiv: DEFAULT_CATEGORIES kept as fallback; __init__ prefers config.
- rss: FEEDS + AI_KEYWORDS kept as module fallbacks; __init__ prefers config.
  Keywords stay regex form (re.search) as in original.
- github: trending queries moved to config search_terms; fallback retained.
- DELETE reddit_proof.py: standalone PoC v5 at repo root, own main()+init_db()
  + direct INSERT OR REPLACE, NOT in cron, NOT imported anywhere -> dead
  code. Also removes its byte-duplicate SUBREDDITS.

NOTE: fallback class constants remain intentionally (issue #7 cut #5: safe
rollout). Curation VALUES now live in one file; the constants are inert
unless config/queries.json is missing.

Verified: all adapters compile; config loads (HN 46 kw, RSS 10 feeds);
full dry-run fetches all 6 sources; grep confirms HN internal dup list gone.

adapters/arxiv.py

@@ -32,7 +32,7 @@ import xml.etree.ElementTree as ET
 from datetime import datetime, timedelta, timezone
 from html import unescape
 
-from adapters import SourceAdapter
+from adapters import SourceAdapter, source_config
 
 # arXiv API
 ARXIV_API = "http://export.arxiv.org/api/query"
@@ -45,12 +45,8 @@ class ArxivAdapter(SourceAdapter):
     DEFAULT_CATEGORIES = ["cs.AI", "cs.LG", "cs.CL"]
 
     def __init__(self, categories=None, rate_limit=3):
-        """
-        Args:
-            categories: List of arXiv categories. Default: cs.AI, cs.LG, cs.CL
-            rate_limit: Seconds between API calls (default 3).
-        """
-        self.categories = categories or self.DEFAULT_CATEGORIES
+        cfg = source_config("arxiv")
+        self.categories = categories or cfg.get("categories") or self.DEFAULT_CATEGORIES
         self.rate_limit = rate_limit
 
     def name(self) -> str: