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.

website/docs/user-guide/configuration.md

@@ -1264,6 +1264,7 @@ display:
     enabled: false
     fields: ["model", "context_pct", "cwd"]
   file_mutation_verifier: true    # Append an advisory footer when write_file/patch calls failed this turn
+  credits_notices: true   # Nous credits status-bar notices (usage bands, grant-spent, depleted). false = silence them; /usage still works
   language: en            # UI language for static messages (approval prompts, some gateway replies). en | zh | zh-hant | ja | de | es | fr | tr | uk | af | ko | it | ga | pt | ru | hu
 ```