fix(graperank): don't create the operator master for a read-only follower crawl

`amy graperank followers <observer>` (no --relay) assembles the relay universe
via allKnownRelays, which read the reachability cache through ctx.reachability —
and that lazily derives the monitor key from the operator master, forcing a
passphrase prompt (or failing on a fresh machine) purely to READ kind:30166
records. The follower crawl signs nothing, so this defeats its anonymous path.

Read the reachability snapshot with a throwaway signer instead (snapshot() only
reads; the signer is used solely for writes) — the same trick `graperank status`
already uses to stay side-effect-free.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Xc3Wm4qVCrAGvSAotTUVt4

cli/src/main/kotlin/com/vitorpamplona/amethyst/cli/commands/GrapeRankCommand.kt

@@ -697,7 +697,12 @@ object GrapeRankCommand {
         ctx: Context,
         args: Args,
     ): Set<NormalizedRelayUrl> {
-        val reach = ctx.reachability.snapshot()
+        // Read the reachability records (kind:30166) with a throwaway signer, NOT
+        // ctx.reachability — the latter derives the monitor key and would CREATE the
+        // operator master (a passphrase prompt) purely to read a cache. The follower
+        // crawl never signs, so keep it truly account-and-key-free. snapshot() only
+        // reads; the signer is used solely for writes. Same trick as `status`.
+        val reach = RelayReachabilityStore(store = ctx.store, signer = NostrSignerInternal(KeyPair())).snapshot()
         val relays = LinkedHashSet<NormalizedRelayUrl>()
         relays.addAll(reach.live)
         // Every advertised outbox/inbox relay in the store — the homes of the users