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

scripts/gen-doc-graphs.ts

@@ -137,6 +137,15 @@ const SERVICE_ROLES: ServiceRole[] = [
     consumers: ['agent-loop', 'tool-bash', 'hooks-claude', 'hooks-codex', 'session-query', 'session-query-sqlite'],
     note: 'Backends persist the same SessionEvent vocabulary; apps choose a backend at composition time.',
   },
+  {
+    key: 'settings',
+    pkg: 'settings',
+    title: 'User-settings seam',
+    mode: 'seam',
+    implementations: ['settings-local'],
+    consumers: [],
+    note: 'Plugins register namespace schemas and resolve layered values; providers store the raw document. No production consumer is migrated yet.',
+  },
   {
     key: 'telemetry',
     pkg: 'session-telemetry',