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>

package.json

@@ -0,0 +1,28 @@
+{
+  "name": "keep",
+  "version": "0.1.0",
+  "license": "MIT",
+  "type": "module",
+  "bin": {
+    "keep": "./dist/cli/index.js"
+  },
+  "scripts": {
+    "dev:server": "tsx watch --env-file=.env src/server/index.ts",
+    "cli": "tsx src/cli/index.ts",
+    "build": "tsc",
+    "start": "node --env-file=.env dist/server/index.js"
+  },
+  "dependencies": {
+    "express": "^4.19.2",
+    "better-sqlite3": "^11.3.0",
+    "libsodium-wrappers": "^0.7.15"
+  },
+  "devDependencies": {
+    "@types/express": "^4.17.21",
+    "@types/node": "^22.0.0",
+    "@types/better-sqlite3": "^7.6.11",
+    "@types/libsodium-wrappers": "^0.7.14",
+    "tsx": "^4.15.0",
+    "typescript": "^5.5.0"
+  }
+}