fix(credits): suppress usage gauge when top-up funds exist + add display.credits_notices toggle (#44716)

The subscription-cap usage gauge (50/75/90% bands) ignored purchased
(top-up) credits: a sub user with top-up funds got a sticky warn banner
at 90% of their cap — permanently at >=100%, alongside grant_spent —
despite being fully able to keep inferencing. The cap is the wrong
denominator for an account that can keep spending.

- evaluate_credits_notices: purchased_micros > 0 suppresses the usage
  band (grant_spent already covers the cap-reached + top-up case with
  the remaining balance). A top-up landing mid-session clears any
  showing band; spending top-up down to 0 resumes the gauge.
- New display.credits_notices config (default true): false silences all
  credits notices. State capture and /usage are unaffected. Read once
  per agent (cached) in _emit_credits_notices, fail-open true.
- Docs: configuration.md display block.

agent/credits_tracker.py

@@ -286,6 +286,16 @@ def evaluate_credits_notices(
         for band in CREDITS_USAGE_BANDS:  # ascending → last match wins = highest
             if uf >= band[0]:
                 current_band = band
+    # Top-up suppression: when the account holds purchased (top-up) credits,
+    # the subscription-cap gauge is the wrong denominator — warning "90% used"
+    # at a user sitting on $50 of top-up is noise (and it previously stuck
+    # PERMANENTLY alongside grant_spent at >=100%). Suppress the usage band
+    # entirely; the cap-reached case is covered by the grant_spent info notice
+    # below, which already names the remaining top-up balance. A top-up landing
+    # mid-session flips current_band → None and the clear path below removes
+    # any showing band line.
+    if state.purchased_micros > 0:
+        current_band = None
     grant_cond = (
         state.denominator_kind == "subscription_cap"
         and uf is not None