Improve signal quality (Exa news category) and remove mock fallback

Exa harvester:
- EXA_CATEGORY=news (default) restricts to news articles, excluding evergreen
  careers/marketing landing pages that were producing irrelevant signals.
- Event-focused query (acquisition/earnings/launch/leadership), dropped
  hiring/career terms that pulled recruiting pages.
- Domain scoping now OFF by default (broad-web news about the account beats
  first-party careers pages); add EXA_EXCLUDE_DOMAINS and loop --domain-filter.

LLM layer:
- Remove the mock reasoner entirely (no provider => loop refuses to run).
- On HTTP 429, WAIT (LLM_RATELIMIT_WAIT_SECONDS, default 900s; honors
  Retry-After) and retry up to LLM_RATELIMIT_MAX_RETRIES (24) instead of
  degrading to mock output.
- If the LLM is ultimately unavailable for an account, skip it and leave its
  signals unconsumed (retried next run) — never fabricate briefs.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

sales-agent/run.py

@@ -23,6 +23,8 @@ from src.memory import store, feedback
 def cmd_loop(args):
     if args.no_domain_filter:
         config.EXA_DOMAIN_FILTER = False  # per-run override
+    if args.domain_filter:
+        config.EXA_DOMAIN_FILTER = True
     pipeline.run_all(tier=args.tier, account_id=args.account)
     print("\n--- digest preview ---")
     print(digest_mod.write_digest())
@@ -85,7 +87,9 @@ def main(argv=None):
     p_loop.add_argument("--account", help="single account id")
     p_loop.add_argument("--tier", choices=["strategic", "enterprise", "growth"])
     p_loop.add_argument("--no-domain-filter", action="store_true",
-                        help="include broad third-party news instead of scoping Exa to the account domain")
+                        help="broad-web news about the account (default behavior)")
+    p_loop.add_argument("--domain-filter", action="store_true",
+                        help="scope Exa to the account's own domain (newsroom/press only)")
     p_loop.set_defaults(func=cmd_loop)
 
     p_dig = sub.add_parser("digest", help="Print the daily intelligence digest")