fix(admin): make create_new_key idempotent — never clobber an existing key

`create_new_key` unconditionally generated a fresh keypair and let
`saveEncrypted` overwrite `config.keys[keyName]` on disk. So calling it
with a name that already exists SILENTLY DESTROYED the in-use signing
key: the old encrypted nsec was overwritten, surviving only in the
running process's in-memory Backend until the next restart, then gone.

This breaks the contract callers already rely on. spirekeeper's
`pair_spire` re-pairs a machine through the same `spire-<id>` keyName and
documents the assumption verbatim — "create_new_key is idempotent —
returns the existing key if the name is taken" (pairing.py). It wasn't:
a re-pair rotated the machine's spire identity and orphaned everything
bound to the old key (tokens, beacons, wallet routing), unrecoverably.

Guard at the top of the command: if a key with this name already exists,
recover and return it instead of generating + overwriting.
  - no `_nsec`: decrypt the existing entry with the supplied passphrase
    and return its npub (the idempotent re-pair path).
  - explicit `_nsec`, or an unrecognized/!iv/!data entry, or a passphrase
    that doesn't decrypt the existing key: throw rather than overwrite —
    destroying a key must never be a silent side effect of "create".

Functionally verified on the dev bunker: a repeat `/pair` for an
already-paired machine now returns the existing spire pubkey with the
on-disk key entry unchanged, where before it minted a new identity and
clobbered the old blob.

src/daemon/admin/commands/create_new_key.ts

@@ -2,6 +2,8 @@ import NDK, { NDKEvent, NDKPrivateKeySigner, NDKRpcRequest, type NostrEvent } fr
 import AdminInterface from "../index.js";
 import { NIP46_ADMIN_RESPONSE_KIND } from "../kinds.js";
 import { saveEncrypted } from "../../../commands/add.js";
+import { getCurrentConfig } from "../../../config/index.js";
+import { decryptNsec } from "../../../config/keys.js";
 import { setupSkeletonProfile } from "../../lib/profile.js";
 
 export default async function createNewKey(admin: AdminInterface, req: NDKRpcRequest) {
@@ -10,6 +12,42 @@ export default async function createNewKey(admin: AdminInterface, req: NDKRpcReq
     if (!keyName || !passphrase) throw new Error("Invalid params");
     if (!admin.loadNsec) throw new Error("No unlockKey method");
 
+    // Idempotency guard. Callers re-pair a machine through the same keyName
+    // (e.g. spirekeeper's `spire-<id>`) and rely on create_new_key being
+    // idempotent — "returns the existing key if the name is taken". Without
+    // this guard the command unconditionally generated a FRESH key and
+    // `saveEncrypted` overwrote the existing encrypted blob on disk, silently
+    // destroying the in-use signing identity (unrecoverable — the old nsec then
+    // survives only in the running process until the next restart). NEVER
+    // generate-and-overwrite an existing name: recover and return it instead.
+    const currentConfig = await getCurrentConfig(admin.configFile);
+    const existing = currentConfig.keys?.[keyName];
+    if (existing) {
+        if (_nsec) {
+            throw new Error(
+                `key '${keyName}' already exists; refusing to overwrite it with an ` +
+                `imported nsec — delete it explicitly first if replacement is intended`,
+            );
+        }
+        if (!existing.iv || !existing.data) {
+            throw new Error(
+                `key '${keyName}' already exists in an unrecognized form; refusing to overwrite`,
+            );
+        }
+        let existingNsec: string;
+        try {
+            existingNsec = decryptNsec(existing.iv, existing.data, passphrase);
+        } catch (e: any) {
+            throw new Error(
+                `key '${keyName}' already exists but the supplied passphrase did not ` +
+                `decrypt it; refusing to overwrite (${e.message})`,
+            );
+        }
+        const existingUser = await new NDKPrivateKeySigner(existingNsec).user();
+        const result = JSON.stringify({ npub: existingUser.npub });
+        return admin.rpc.sendResponse(req.id, req.pubkey, result, NIP46_ADMIN_RESPONSE_KIND);
+    }
+
     let key;
 
     if (_nsec) {