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/settings/settings-local/README.md

@@ -0,0 +1,36 @@
+# @deepseek-ai/dsh-settings-local
+
+English | [中文](README.zh.md)
+
+File-backed settings provider. One YAML or JSON document carries every namespace section; external edits hot-publish through `ctx.settings`, and `update()` writes back atomically while preserving the user's YAML comments and any section owned by a plugin that is not currently loaded.
+
+## Config
+
+| Field | Meaning | Default |
+|---|---|---|
+| `path` | Settings document path; extension picks the format (`.yaml`/`.yml`/`.json`) | `settings.yaml` under the harness home |
+| `dshHome` | Harness home used when `path` is omitted | `$DSH_HOME` or `~/.dsh` |
+| `watch` | Watch the document and hot-publish external edits | `true` |
+| `debounceMs` | Watcher write-settle window in milliseconds | `100` |
+
+Defaulting is one explicit `resolveSpec(config)` step; an unsupported extension fails at load.
+
+## Behavior
+
+- **Boot fails loud, reload keeps last-good.** An existing-but-invalid document fails plugin load; once live, an unreadable or unparsable edit warns and keeps the last good sections. A missing document resolves every namespace from defaults and `base`; deleting it publishes the same empty state.
+- **Write-back is atomic and owner-only.** `persist` writes `<path>.tmp` with mode `0600` and renames over the target. YAML writes patch one namespace in the comment-preserving document; JSON re-serializes.
+- **Self-write suppression by content.** The provider caches the last good text; a watcher event whose content equals the cache (its own write included) is a no-op.
+
+## Model Experience
+
+Indirectly, through consumers of `ctx.settings`: this provider only stores and publishes namespace sections, and each consumer's own surface documents any model effect.
+
+#### KV Cache effect
+
+No direct invalidation; the consuming plugin owns any request-prefix changes.
+
+## Known Limitations and Deferred Work
+
+- **No cross-process write lock** — concurrent writers (for example TUI and web on one home) converge by atomic replace plus watcher reload, last write wins; a lockfile is deferred until real contention shows up.
+- **Comment preservation is YAML-only** — JSON documents re-serialize without comments (JSON has none) and lose hand formatting.
+- **No value indirection** — sections hold literal values; `${env:VAR}`-style references for secrets are a deferred seam-level feature.