feat: add production capability foundations

src/adapter/outbound/notification/CLAUDE.md

@@ -12,10 +12,28 @@ Package root: `dev.caskeleton.adapter.outbound.notification`.
 
 ## Responsibility
 
-- Implement notification provider routing and provider-specific Slack/email clients behind ports.
-- Own provider settings, technical fallback, and provider adaptation.
+- Preserve the current raw notification router/provider seams only as the `R0 legacy` compatibility
+  baseline until the reviewed canonical cutover removes them.
+- Implement future provider protocols behind application-owned ports without leaking SDK, transport,
+  bootstrap, or persistence types.
+- Own provider settings, technical fallback mechanics, and provider adaptation; application policy
+  owns mode, eligibility, retry/fallback decisions and business failure semantics.
 - Reuse `adapter:outbound:support` for shared outbound concerns.
 
+## Current R0 freeze
+
+- `RoutingNotifier` performs route-list fan-out over `(Channel, providerId)`.
+- `FailOpenNotificationProvider` applies one global fail-open rule.
+- `google-email`/`GoogleEmailClient` and `slack-webhook`/`SlackClient` are fake-only extension seams,
+  not production integrations or qualified provider cards.
+- Checked-in provider selector keys drift from the router's `routes` + provider `enabled` grammar.
+  Preserve and document that drift until the canonical graph replaces it; do not silently reinterpret
+  the old keys.
+- There are no feature/application production consumers and no real-provider, durable, receipt,
+  security, load, or rotation evidence.
+- The exact legacy deletion inventory lives in [README.md](README.md). Do not add behavior to those
+  classes while building their canonical replacements.
+
 ## Boundaries
 
 - Allowed dependency edges come only from the module's