Sprint 3: Anti-bot retrieval layer + metrics module

Anti-bot changes (all 6 adapters):
- Browser-grade User-Agent rotation (Chrome/Firefox on Linux/Windows)
- Shared browser_headers() with Accept, Accept-Language, DNT
- Session-consistent UA fingerprint (picked once, not per-request)
- jitter_sleep() replaces fixed time.sleep() on all adapters
- Exponential backoff on 429/503 already on reddit, now consistent

New shared module:
- adapters/__init__.py: browser_user_agent(), browser_headers(), jitter_sleep()
- adapters/_http.py: HTTPClient class for future browser-mode adapters

Metrics module (from Sprint 2 carry):
- oracle/metrics.py: MetricsRun for log_adapter/log_verdicts/log_scores
- oracle/weekly.py: SYSTEM HEALTH section wired to adapter_health
- oracle/cli.py: metrics subparser with --adapters/--publish/--scores/--alerts

Before: bot signatures like 'ai-oracle/0.1', 'python:athena:v0.1'
After: 'Mozilla/5.0 (X11; Linux x86_64; rv:139.0) Gecko/20100101 Firefox/139.0'

adapters/reddit.py

@@ -26,7 +26,7 @@ import xml.etree.ElementTree as ET
 from datetime import datetime, timezone
 from html import unescape
 
-from adapters import SourceAdapter
+from adapters import SourceAdapter, browser_user_agent, jitter_sleep
 from adapters._store import true_first_seen, upsert_entries
 
 
@@ -62,7 +62,7 @@ class RedditAdapter(SourceAdapter):
         """
         self.subreddits = subreddits or self.DEFAULT_SUBREDDITS
         self.rate_limit = rate_limit
-        self.user_agent = user_agent or "python:ai-oracle:v0.1 (by tony_tech)"
+        self.user_agent = user_agent or browser_user_agent()
 
     def name(self) -> str:
         return "reddit"
@@ -212,7 +212,7 @@ class RedditAdapter(SourceAdapter):
                 if e.code in (429, 403, 404):
                     return []  # JSON endpoint blocked, fall back to RSS
                 if attempt < 1:
-                    time.sleep(5)
+                    jitter_sleep(5)
                     continue
                 return []
             except Exception:
@@ -417,7 +417,7 @@ class RedditAdapter(SourceAdapter):
         if not json_worked:
             print("  JSON endpoints blocked, using RSS fallback")
             # Brief cooldown before RSS barrage
-            time.sleep(3)
+            jitter_sleep(3)
             for sub in self.subreddits:
                 entries = self._fetch_rss(sub)
                 for e in entries: