feat(settings): add user-settings seam (ctx.settings) + file provider

Two-package capability family mirroring session-persistence/:

- dsh-settings: abstract Settings service — namespace registry with
  caller-fiber effect registrations, layered resolution (schema defaults
  < composition base < user document), schemastery validation,
  per-namespace deep-equal commit detection, and the settings/updated
  event. Boot/registration validation fails loud; provider publishes
  keep last-good per namespace.
- dsh-settings-local: settings.yaml/.json provider — resolveSpec
  defaulting to $DSH_HOME/settings.yaml, chokidar hot reload,
  content-equality self-write suppression, atomic 0600 tmp+rename
  writes, comment-preserving YAML namespace patching.

Consumers register inside ctx.inject(['settings'], …), so every
composition works unchanged without a mounted provider. Real Loader +
Include composition test proves cordis.yml boot and external-edit hot
propagation; HMR disposal test proves registry cleanup. Both packages
hold per-file 100% coverage.

Doc budgets rise 1705→1710 (AGENTS.md) and 835→845 (packages/README.md):
one structural line per file for the new package group.

Agent Note: .agents/notes/implemented/architecture/2026-07-28-user-settings-seam.md

packages/README.zh.md

@@ -37,6 +37,7 @@
 | [`session-persistence/`](session-persistence/README.md) | 持久化能力系列：seam + JSONL/SQLite 后端 | 产品：稳定表面 |
 | [`session-query/`](session-query/README.md) | 会话检索系列：逻辑语料库、有界读取、血缘、事件关系、语义过滤和 SQLite 全文搜索 | 产品：稳定表面 |
 | [`session-title/`](session-title/README.md) | 日志支撑的会话标题：回退服务、共享 LLM 策略和选用提供方 | 产品：稳定表面 |
+| [`settings/`](settings/README.md) | 用户设置能力族：seam + 文件 provider | 产品：稳定表面 |
 | [`telemetry/`](telemetry/README.md) | 会话上报：捕获／脱敏 seam、OTel 后端 | 产品：稳定表面 |
 | [`storage/`](storage/README.md) | 非会话存储中枢 + 后端 + 领域形式 | 产品：稳定表面 |
 | [`workspace/`](workspace/README.md) | Workspace 实体 | 产品：稳定表面 |