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/github.py

@@ -17,7 +17,7 @@ import urllib.error
 import urllib.parse
 from datetime import datetime, timedelta, timezone
 
-from adapters import SourceAdapter
+from adapters import SourceAdapter, browser_user_agent, jitter_sleep
 from adapters._store import true_first_seen, upsert_entries
 
 
@@ -37,7 +37,7 @@ class GitHubAdapter(SourceAdapter):
     def _headers(self):
         headers = {
             "Accept": "application/vnd.github.v3+json",
-            "User-Agent": "ai-oracle/0.1",
+            "User-Agent": browser_user_agent(),
         }
         if self.token:
             headers["Authorization"] = f"token {self.token}"
@@ -166,7 +166,7 @@ class GitHubAdapter(SourceAdapter):
             ]:
                 batch = self._search_repos(q, sort="stars", per_page=30)
                 repos.extend(batch)
-                time.sleep(1)  # polite spacing
+                jitter_sleep(1)  # polite spacing
 
             # Deduplicate by full_name
             seen = set()
@@ -234,7 +234,7 @@ class GitHubAdapter(SourceAdapter):
             readme_text = ""
             if owner and repo_name and idx < readme_budget:
                 readme_text = self._get_readme(f"https://api.github.com/repos/{owner}/{repo_name}/readme")
-                time.sleep(0.5)  # polite spacing between README fetches
+                jitter_sleep(0.5)  # polite spacing between README fetches
 
             # Structured metadata
             stars = repo.get("stargazers_count", 0)