fix(cli): add per-tier budget utilization breakdown to context show

Extends the budget utilization summary in `context show` to display
hot/warm/cold tier utilization percentages individually, satisfying
the spec requirement for a per-tier breakdown. Previously only hot
tier utilization was shown.

- Hot tier: tokens used vs. max_tokens_hot budget
- Warm tier: fragment count vs. max_decisions_warm budget
- Cold tier: fragment count vs. max_decisions_cold budget

Also updates Robot Framework helper to verify per-tier breakdown
is present in output, and updates CHANGELOG/CONTRIBUTORS.

ISSUES CLOSED: #9586

robot/helper_acms_context_cli.py

@@ -150,6 +150,14 @@ def _cmd_context_show() -> int:
     if expected not in output:
         print(f"acms-fail: expected {expected!r} in output\n{output}")
         return 1
+    # Verify per-tier budget utilization breakdown is present
+    for tier_label in ("Hot tier", "Warm tier", "Cold tier"):
+        if tier_label not in output:
+            print(
+                f"acms-fail: expected per-tier breakdown"
+                f" '{tier_label}' in output\n{output}"
+            )
+            return 1
     print("acms-context-show-ok")
     return 0