fix(secrets): list via GET ?list=true — the LIST verb dies at the proxy

Found by the readiness probe added in the previous commit, on its first
run against production: `mcpctl status` reported

  Secrets:    bao-k8s* ✗ auth failed: OpenBao list: HTTP 400 Bad Request

That is a real bug, not a probe artifact. `bao-k8s` is configured with
the public ingress URL, and Cilium's ingress Envoy rejects the
non-standard LIST HTTP method outright. Verified live from the mcpd pod:

  LIST   https://bao.ad.itaz.eu/v1/secret/metadata/mcpctl/            -> 400 Bad Request
  GET    https://bao.ad.itaz.eu/v1/secret/metadata/mcpctl/?list=true  -> 403 permission denied (bogus token, i.e. reached bao)
  LIST   http://openbao.openbao.svc:8200/... (ClusterIP, no Envoy)    -> 403 permission denied

So the verb was fine against bao and fatal through the ingress. OpenBao
accepts both forms and documents the GET form for exactly this reason.

This was never noticed because nothing called list() in production —
it backs `mcpctl migrate secrets`, which would have failed with an opaque
HTTP 400 against any ingress-fronted backend.

Also adds the per-server scoping primitives Phase 2 needs, with tests:
buildServerSecretPolicyHcl (no wildcards, read-only, stable under
reordering), buildServerProvisioningPolicyHcl (prefix-confined so mcpd
cannot grant itself more than it holds), ensureKubernetesAuthRole /
delete helpers, the driver-level ensureServerIdentity/removeServerIdentity
capability, and ServerIdentityService.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018vybEitX4FykeMatKe5Xki

src/mcpd/tests/secret-backends.test.ts

@@ -87,9 +87,9 @@ describe('OpenBaoDriver', () => {
     await expect(driver.delete({ name: 'gone', externalRef: '' })).resolves.toBeUndefined();
   });
 
-  it('list returns names from the metadata LIST call', async () => {
+  it('list returns names from the metadata listing', async () => {
     const fetchFn = makeFetch([{
-      url: /\/v1\/secret\/metadata\/mcpctl\/$/,
+      url: /\/v1\/secret\/metadata\/mcpctl\/\?list=true$/,
       status: 200,
       body: { data: { keys: ['token1', 'token2', 'sub-folder/'] } },
     }]);
@@ -98,6 +98,10 @@ describe('OpenBaoDriver', () => {
       { fetch: fetchFn as unknown as typeof fetch, secretRefResolver: resolver },
     );
     const result = await driver.list();
+    // GET ?list=true rather than the LIST verb: proxies (Cilium's ingress
+    // Envoy among them) answer a bare 400 to the non-standard method.
+    const [, listInit] = fetchFn.mock.calls[0] as [unknown, RequestInit];
+    expect(listInit.method).toBe('GET');
     // Sub-folders (trailing slash) are excluded; only leaf keys are returned.
     expect(result).toEqual([
       { name: 'token1', externalRef: 'secret/mcpctl/token1' },