revert(sandbox): withdraw the credential-document read denial

The `readDenyPaths` policy field shipped in the previous commit broke Linux
confinement outright. bwrap has to create the `/dev/null` bind's mount point
inside a tree its own profile has already made read-only, so it refused the
entire confinement whenever the parent directory was absent — every host that
has not stored a credential yet, including a fresh install:

  bwrap: Can't mkdir parents for /home/runner/.dsh/.env: Read-only file system

which the executor correctly classifies as SANDBOX_UNAVAILABLE, so every
confined bash call failed closed. Landlock cannot subtract from its own `/`
read grant, so it reported `partial` enforcement on every confined call for a
file it never hid, with no way to switch the denial off (schemastery fills an
omitted array with `[]`, so empty and omitted were indistinguishable).

A protection that breaks confinement where it works and misreports it where it
does not is worse than a documented absence. Revert the field, both expressible
backends, the enforcement downgrade, and the policy default; state the residue
plainly in the credentials-local READMEs — file mode stops other OS users, not
the model — and keep the OS-keychain provider recorded as the real answer.

The narrower discipline stands: no surface hoists the credential document into
`process.env`, and the model is never handed a resolved path to it.

packages/sandbox/sandbox-policy/package.json

@@ -28,7 +28,6 @@
   "license": "BSD-3-Clause",
   "peerDependencies": {
     "@deepseek-ai/dsh-invariants": "^0.0.1",
-    "@deepseek-ai/dsh-paths": "^0.0.1",
     "@deepseek-ai/dsh-sandbox": "^0.0.1",
     "@deepseek-ai/dsh-session": "^0.0.1",
     "cordis": "^4.0.0-rc.7"
@@ -38,7 +37,6 @@
   },
   "devDependencies": {
     "@deepseek-ai/dsh-invariants": "workspace:^",
-    "@deepseek-ai/dsh-paths": "workspace:^",
     "@deepseek-ai/dsh-sandbox": "workspace:^",
     "@deepseek-ai/dsh-session": "workspace:^",
     "cordis": "^4.0.0-rc.7"