docs(protocol): rewrite config-resolution onto the shipped SettingsService contract (#5888) (#6031)

The page taught a `context.config` API that has zero implementation:
`context.config` itself, `setTenant()`, `setUserPreference()`,
`admin.setTenantConfig()`, and the `objectstack_tenant_config` /
`objectstack_user_preferences` / `objectstack_secrets` tables all return
zero hits across packages/, apps/ and examples/. The whole-page audit found
the gap is wider than the three symbols the issue named: YAML/JSON config
files, NODE_ENV-selected config files, `defineStack({ database, http,
features, secrets })`, deep-merge semantics, and bundled external secret
managers are equally unimplemented.

The capability itself DOES exist, under a different shape: `SettingsService`
(@objectstack/service-settings, ADR-0007), backed by `sys_setting` /
`sys_secret` and served over /api/settings. So this is a rewrite onto the
real signatures rather than a deletion.

Corrections of record:
- Cascade is five layers (env > global > tenant > user > default), not six.
  `global` was missing entirely; `runtime` and `file` do not exist.
- First non-null layer wins. There is no deep merge and no array merge.
- Scope is DECLARED by the manifest, never chosen by the caller — which is
  why setTenant()/setUserPreference() cannot exist as written.
- `tenant_id` is not a column on `sys_setting`. Platform-wide the tenant
  identity is the organization: sessions carry `organizationId`, and where an
  object does declare `tenant_id` it is a lookup to `sys_organization`.
- Arrays concat on stack composition — the opposite of what the page claimed.

The central manifest example now carries an `{/* os:check */}` marker, so it
is type-checked against the built spec by check:skill-examples and cannot rot
silently the way this page did.

Refs #5888

Co-authored-by: Claude <noreply@anthropic.com>

Author: hotlong