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/hackernews.py

@@ -20,7 +20,7 @@ import urllib.request
 import urllib.error
 from datetime import datetime, timezone
 
-from adapters import SourceAdapter
+from adapters import SourceAdapter, browser_user_agent, jitter_sleep
 from adapters._store import true_first_seen, upsert_entries
 
 
@@ -49,7 +49,7 @@ class HackerNewsAdapter(SourceAdapter):
     ]
 
     def __init__(self, user_agent=None):
-        self.user_agent = user_agent or "python:athena:v0.1 (by tony_tech)"
+        self.user_agent = user_agent or browser_user_agent()
 
     def name(self) -> str:
         return "hackernews"
@@ -57,7 +57,7 @@ class HackerNewsAdapter(SourceAdapter):
     def _request(self, path: str, max_retries: int = 2) -> dict | list | None:
         """Make a GET request to the HN Firebase API."""
         url = f"{self.BASE}{path}"
-        req = urllib.request.Request(url, headers={"User-Agent": self.user_agent})
+        req = urllib.request.Request(url, headers={"User-Agent": browser_user_agent()})
 
         for attempt in range(max_retries + 1):
             try:
@@ -199,7 +199,7 @@ class HackerNewsAdapter(SourceAdapter):
             if item and item.get("type") == "story" and item.get("title"):
                 stories.append(item)
             if idx % 20 == 19:  # polite spacing every 20 requests
-                time.sleep(1)
+                jitter_sleep(1)
 
         # Filter for AI relevance
         ai_stories = [s for s in stories if self._is_ai_relevant(s.get("title", ""))]