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

docs/cordis-catalog/events.md

@@ -640,6 +640,28 @@ Types: [Scoped](../core-data-structures/scope.md) · [Session](../core-data-stru
 
 Source: [`packages/core/session/src/index.ts:102`](../../packages/core/session/src/index.ts)
 
+## `settings/*`
+
+### `settings/updated` — emit
+
+Committed change to one registered namespace's resolved value. Emitted after the provider persisted (for `update`) or published (`provider`) the change; never emitted when the resolved value is deep-equal.
+
+```ts cordis-catalog
+/**
+ * Committed change to one registered namespace's resolved value. Emitted
+ * after the provider persisted (for `update`) or published (`provider`)
+ * the change; never emitted when the resolved value is deep-equal.
+ * @param ns - the namespace whose resolved value changed.
+ * @param next - the new resolved value.
+ * @param prev - the previous resolved value.
+ * @param source - whether the change entered through `update()` or the provider.
+ * @mode emit
+ */
+'settings/updated'(ns: SettingsNamespace, next: unknown, prev: unknown, source: SettingsUpdateSource): void
+```
+
+Source: [`packages/settings/settings/src/index.ts:90`](../../packages/settings/settings/src/index.ts)
+
 ## `slash/*`
 
 ### `slash/input-begin-command` — bail

docs/cordis-catalog/services.md

@@ -1388,6 +1388,48 @@ Types: [Session](../core-data-structures/session.md) · [SessionTitleProvider](.
 
 Source: [`packages/session-title/session-title/src/index.ts:232`](../../packages/session-title/session-title/src/index.ts)
 
+## `ctx.settings` — `Settings` (abstract seam)
+
+Abstract settings service. Providers implement raw-document storage (`load`/`persist`) and push external changes through Settings.publish; the base class owns namespace registration, resolution, validation, change detection, and the `settings/updated` commit event.
+
+```ts cordis-catalog
+/**
+ * Register a namespace schema and receive its owner scope. The registration
+ * is an effect on the calling plugin's fiber: disposing that fiber removes
+ * the namespace and its observers. An invalid stored section fails the
+ * registration itself — the earliest point where the schema can judge it.
+ * @param ns - unique namespace; duplicate registration fails loud.
+ * @param schema - schemastery schema resolving this namespace's value.
+ * @param options - composition `base` layer and effect timing.
+ * @returns the owner scope for reads, observation, and updates.
+ */
+register<T>(ns: SettingsNamespace, schema: z<T>, options?: SettingsRegisterOptions<T>): SettingsScope<T>
+
+/**
+ * Describe every registered namespace for configuration surfaces.
+ * @returns one descriptor per registered namespace, in registration order.
+ */
+describe(): SettingsDescriptor[]
+
+/**
+ * Read one registered namespace's resolved value.
+ * @param ns - the namespace to read.
+ * @returns the resolved value, or `undefined` while unregistered.
+ */
+get(ns: SettingsNamespace): unknown
+
+/**
+ * Merge a patch into one registered namespace's user layer, validate the
+ * resolved candidate, persist through the provider, then commit and emit.
+ * A validation failure rejects before anything is persisted.
+ * @param ns - the registered namespace to update.
+ * @param patch - plain-object patch over the user section.
+ */
+async update(ns: SettingsNamespace, patch: object): Promise<void>
+```
+
+Source: [`packages/settings/settings/src/index.ts:140`](../../packages/settings/settings/src/index.ts)
+
 ## `ctx.skills` — `SkillService`
 
 Registry of skill providers. It merges provider catalogs with stable first-wins duplicate handling, exposes sorted model-visible summaries, and loads full skill bodies on demand.