feat(llm): mask API keys in UI responses and prevent masked-key leakage in fetch/test/save payloads

- Add mask_api_key() and is_masked_or_placeholder() to llm_provider service
- Return masked keys in all provider CRUD endpoints
- Reject masked/placeholder keys in fetch_models and test_provider_config
- Show masked key with Change button in ProviderConfig.svelte edit form
- Exclude masked keys from fetch-models, test, and submit payloads on frontend
- Update semantics-core skill with clarified complexity tier rules
- Switch agent modes from subagent to all

.opencode/agents/qa-tester.md

@@ -1,12 +1,12 @@
 ---
 description: QA & Semantic Auditor — orthogonal verification, contract validation, code review, and regression defense for Python (pytest) and Svelte (vitest).
-mode: subagent
-model: opencode-go/mimo-v2.5
+mode: all
+model: opencode-go/kimi-k2.6
 temperature: 0.1
 permission:
   edit: allow
   bash: allow
-  browser: deny
+  browser: allow
 steps: 80
 color: accent
 ---