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/__init__.py

@@ -1,10 +1,55 @@
 """Source adapters for AI Research Oracle."""
 
+import random
 import urllib.request
 import urllib.error
 import time
 from abc import ABC, abstractmethod
 
+# --- Browser-grade headers (anti-bot) ---
+# Picked once per session so fingerprint stays consistent.
+_SESSION_UA: str | None = None
+
+_BROWSER_UAS = [
+    "Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/138.0.0.0 Safari/537.36",
+    "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/138.0.0.0 Safari/537.36",
+    "Mozilla/5.0 (X11; Linux x86_64; rv:139.0) Gecko/20100101 Firefox/139.0",
+    "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:139.0) Gecko/20100101 Firefox/139.0",
+]
+
+
+def browser_user_agent() -> str:
+    """Return a realistic browser User-Agent (same for the session)."""
+    global _SESSION_UA
+    if _SESSION_UA is None:
+        _SESSION_UA = random.choice(_BROWSER_UAS)
+    return _SESSION_UA
+
+
+def browser_headers(api_mode: bool = False) -> dict:
+    """Build realistic browser headers for adapter requests.
+
+    Args:
+        api_mode: If True, use Accept: application/json (for JSON APIs).
+    """
+    headers: dict = {
+        "User-Agent": browser_user_agent(),
+        "Accept-Language": "en-US,en;q=0.9",
+        "Accept-Encoding": "gzip, deflate, br",
+        "DNT": "1",
+    }
+    if api_mode:
+        headers["Accept"] = "application/json, text/json, */*;q=0.8"
+    else:
+        headers["Accept"] = "text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,*/*;q=0.8"
+    return headers
+
+
+def jitter_sleep(base: float = 1.0, range_frac: float = 0.3) -> None:
+    """Sleep for base ± range_frac fraction to break mechanical patterns."""
+    actual = base + base * range_frac * (2 * random.random() - 1)
+    time.sleep(actual)
+
 
 class SourceAdapter(ABC):
     """Base class for all ingestion adapters."""