#20146: fix(agents): use label and model in subagent completion announce

## Summary Prefer `params.label` over agent id in subagent completion announce header. Append child session model name when available (e.g. `claude-sonnet-4-6`). Strip provider prefix from model name (e.g. `anthropic/claude-sonnet-4-6` → `claude-sonnet-4-6`). **Before:** `✅ Subagent main finished` (always "main", no way to tell which task or model) **After:** `✅ Subagent lint-fix (claude-sonnet-4-6) finished` Graceful degradation: no label → agent id, no model → omit parenthetical. ## Changes - `subagent-announce.ts`: Use `params.label || resolveAgentIdFromSessionKey()` for baseName; add `model` param; strip provider prefix from model - `subagent-registry.ts`: Pass `model: entry.model` to announce flow - Tests updated to match new format