feat(credentials): abstract credential seam (ctx.credentials)

References-not-values doctrine: settings carry env-shaped CredentialRefs,
providers own storage. Per-operation resolve, UI-safe describe, fail-loud
set/unset under read-only shadowing, credentials/updated commit event with
a live-service invariant.

packages/credentials/credentials/tsconfig.json

@@ -0,0 +1,24 @@
+{
+  "extends": "../../../tsconfig.base.json",
+  "compilerOptions": {
+    "rootDir": "src",
+    "outDir": "lib/types"
+  },
+  "include": [
+    "src"
+  ],
+  "references": [
+    {
+      "path": "../../../vendor/cosmokit"
+    },
+    {
+      "path": "../../../vendor/cordis"
+    },
+    {
+      "path": "../../util/brand"
+    },
+    {
+      "path": "../../support/invariants"
+    }
+  ]
+}