#14474: feat(config): add agents.defaults.reasoningDefault for default reasoning visibility (#13607)

## Summary Fixes #13607 ## Problem There is no way to set a default reasoning visibility level for agents. Users who want reasoning output (e.g. `medium` or `high`) must set it per-session or per-directive. The `agents.defaults` config supports `thinkingDefault` and `verboseDefault` but not `reasoningDefault`. ## Fix Add `reasoningDefault` to `AgentDefaultsConfig` (type + Zod schema), and wire it into the reasoning level resolution chain in both `get-reply-directives.ts` and `status.ts`, following the same pattern as `thinkingDefault` and `verboseDefault`. ### Resolution order ``` directives.reasoningLevel ?? session.reasoningLevel ?? agentCfg.reasoningDefault ?? "off" ``` ## Reproduction & Verification ### Before fix (main branch): - `agents.defaults` has no `reasoningDefault` field. - `resolvedReasoningLevel` in `get-reply-directives.ts` falls through to `"off"` with no config fallback. - `status.ts` shows reasoning as `off` regardless of any config. ### After fix — All verified: ``` ✓ shows reasoning level from reasoningDefault config (#13607) ✓ defaults reasoning to off when no reasoningDefault is set ... (19 more tests) 21 tests pass (pnpm vitest run src/auto-reply/status.test.ts) ``` ## Testing - ✅ 21 tests pass (`pnpm vitest run src/auto-reply/status.test.ts`) - ✅ Lint passes