feat(credentials): file-backed provider layering process env over $DSH_HOME/.env

Live environment wins read-only (shadowed writes reject instead of
appearing to succeed); the file is the writable source with byte-preserving
line edits, a quoting ladder dotenv reads back verbatim, atomic 0600
writes, wholesale snapshot replacement on reload, and write-drain
teardown.

packages/credentials/credentials-local/tsconfig.json

@@ -0,0 +1,33 @@
+{
+  "extends": "../../../tsconfig.base.json",
+  "compilerOptions": {
+    "rootDir": "src",
+    "outDir": "lib/types"
+  },
+  "include": [
+    "src"
+  ],
+  "references": [
+    {
+      "path": "../../../vendor/cosmokit"
+    },
+    {
+      "path": "../../../vendor/cordis"
+    },
+    {
+      "path": "../../../vendor/schemastery"
+    },
+    {
+      "path": "../../util/atomic-write"
+    },
+    {
+      "path": "../../util/paths"
+    },
+    {
+      "path": "../credentials"
+    },
+    {
+      "path": "../../support/invariants"
+    }
+  ]
+}