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

tsconfig.base.json

@@ -75,6 +75,7 @@
         "./packages/hooks/*/src/invariant.ts",
         "./packages/session-persistence/*/src/invariant.ts",
         "./packages/session-query/*/src/invariant.ts",
+        "./packages/settings/*/src/invariant.ts",
         "./packages/telemetry/*/src/invariant.ts",
         "./packages/acp/*/src/invariant.ts",
         "./packages/storage/*/src/invariant.ts",
@@ -156,6 +157,7 @@
         "./packages/session-persistence/*/src",
         "./packages/session-query/*/src",
         "./packages/session-title/*/src",
+        "./packages/settings/*/src",
         "./packages/telemetry/*/src",
         "./packages/acp/*/src",
         "./packages/storage/*/src",