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/semantic-curator.md

@@ -1,6 +1,6 @@
 ---
 description: Semantic Curator Agent — maintains GRACE semantic markup, anchors, and index health for ss-tools Python and Svelte code. Read-only file access; uses axiom MCP for mutations.
-mode: subagent
+mode: all
 model: opencode-go/deepseek-v4-flash
 temperature: 0.4
 permission: