Implement keep: self-hosted E2E encrypted secrets sync

Server: Express + better-sqlite3 (WAL), multi-recipient key-wrapping
per IMPLEMENTATION.md's design — vaults/recipients/vault_grants/
access_log. Two auth paths: ADMIN_PASSWORD header for recipient
management and revoke (pure metadata operations), signed-request
auth (Ed25519 signature over method+path+timestamp+body-hash) for
push/pull/grant, mirroring the spirit of this project family's other
signed-handshake patterns without naming them.

CLI: identity init/show/set-id, push/pull/grant/log, admin recipient
add/list/remove and revoke. Grant is a client-side crypto operation
(the granter unwraps the vault's current key locally and reseals it
for the new recipient) rather than a server-side operation, since the
server never holds an unwrapped key to grant with.

Verified end-to-end with two independent local identities against a
live server and separately against the built Docker image: register,
push, pull (granted and ungranted), grant without re-pushing, admin
revoke, a subsequent rotation confirming the revoked recipient stays
excluded, and rejection of missing/malformed signed-request auth.

Two real bugs caught during verification, not just written up:
- libsodium-wrappers' published ESM build does a relative import only
  resolvable under bundler-style resolution — broken under plain Node
  ESM. Fixed via createRequire to force the CJS build.
- Express's req.path inside a sub-router is relative to the mount
  point, which would have silently mismatched a client signing the
  full request path. Fixed by verifying against req.originalUrl.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/cli/identityStore.ts

@@ -0,0 +1,53 @@
+import fs from 'node:fs';
+import path from 'node:path';
+import os from 'node:os';
+import { Identity } from '../shared/crypto.js';
+
+const DIR = path.join(os.homedir(), '.keep');
+const FILE = path.join(DIR, 'identity.json');
+
+interface StoredIdentity {
+  seedHex: string;
+  // Assigned by whoever runs `keep recipient add` after registering this
+  // identity's public key — not derivable from the keypair itself, so it
+  // has to be stored once known via `keep identity set-id`.
+  recipientId?: string;
+}
+
+function readStored(): StoredIdentity | null {
+  if (!fs.existsSync(FILE)) return null;
+  return JSON.parse(fs.readFileSync(FILE, 'utf8')) as StoredIdentity;
+}
+
+function writeStored(data: StoredIdentity): void {
+  fs.mkdirSync(DIR, { recursive: true, mode: 0o700 });
+  fs.writeFileSync(FILE, JSON.stringify(data, null, 2), { mode: 0o600 });
+}
+
+export async function loadOrCreateIdentity(): Promise<{ identity: Identity; recipientId: string | null; created: boolean }> {
+  const stored = readStored();
+  if (stored) {
+    return { identity: await Identity.fromSeedHex(stored.seedHex), recipientId: stored.recipientId ?? null, created: false };
+  }
+  const identity = await Identity.generate();
+  writeStored({ seedHex: identity.seedHex });
+  return { identity, recipientId: null, created: true };
+}
+
+export function setRecipientId(recipientId: string): void {
+  const stored = readStored();
+  if (!stored) throw new Error("no local identity — run 'keep identity init' first");
+  writeStored({ ...stored, recipientId });
+}
+
+export async function requireIdentityAndRecipientId(): Promise<{ identity: Identity; recipientId: string }> {
+  const stored = readStored();
+  if (!stored) throw new Error("no local identity — run 'keep identity init' first");
+  if (!stored.recipientId) {
+    throw new Error(
+      "this identity has no recipient id set yet — after an admin registers your public key " +
+      "('keep identity show' to get it), run 'keep identity set-id <recipient-id>' with the id they give you back",
+    );
+  }
+  return { identity: await Identity.fromSeedHex(stored.seedHex), recipientId: stored.recipientId };
+}