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

AGENTS.md

@@ -30,6 +30,7 @@ packages/    @deepseek-ai/dsh-<pkg> workspaces at packages/<group>/<pkg>/
   cordis/      self-referential toolset: the agent inspects/mounts plugins in its own runtime
   hooks/       Claude Code/Codex hook bridges + shared wire-protocol library
   session-persistence/  persistence seam + JSONL/SQLite backends
+  settings/    user-settings seam + file-backed provider
   acp/         automation-only Agent Client Protocol server
   ui/          TUI/JSON-RPC bridges; boot, approval, interaction plugins
   examples/    demo bundles (agent-spine + TUI/CLI/ACP/JSON-RPC bins) leaves load