Spelunk: sign each device in through the browser instead of keeping Spelunk API keys in the vault

A Spelunk API key reaches one organisation, so a server elsewhere - oc-linux in Orange Climate - was
refused before it reached the agent. Each device now signs in to a Spelunk once, under Settings ->
Security -> Spelunk or from a Spelunk host's editor: the OAuth authorization code flow with PKCE for a
native app (RFC 8252), with Spelunk as the authorization server. The browser opens Spelunk's
/api/app-sign-in/authorize with a loopback redirect (the app's own scheme on Android), the person signs
in there however they normally do and allows "DodoSSH on <device>", and the code is redeemed at
/api/app-sign-in/token for an app session token. That token opens terminals in every organisation its
person is in, with the role they have on each server at the time; tunnel tickets send no organisation.

The sign-in stays on the device: sealed in the local cache under the LocalCacheKey (AAD resource type
18, bound to its row), never synced, wiped with the cache, and only ever sent to the Spelunk it came
from. A 401 from Spelunk forgets it; SIGN OUT ends it there and here, and so does signing out of
DodoSSH. Before every Spelunk connection DodoSSH asks its own identity provider whether its sign-in
still stands (a forced refresh, at most once a minute), and an account it refuses is signed out of
DodoSSH and of every Spelunk on the device.

A route names no key any more: Spelunk hosts are written at host schema 11, which an older build shows
read-only and asks to be updated for, and a payload that still names a key credential decodes with the
id skipped. The SpelunkKey item type is gone: SyncEntityType 14, ChangeEntityType 14 and AAD type 17
are retired, the server drops spelunk_key and its change-log rows, and the client cache drops its
cached items, outbox entries and conflicts of that type. A link still never starts a sign-in: from a
Spelunk this device is not signed in to it opens the editor, which offers to.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/DodoSSH.Client.Domain.Tests/HostFactory.cs

@@ -39,18 +39,14 @@ internal static class HostFactory
         BindPort = 1080,
     };
 
-    /// <summary>A credential holding a Spelunk API key, for the hosts reached through Spelunk.</summary>
-    internal static Guid SpelunkKey { get; } = Guid.Parse("0192f0c8-cccc-7c3d-8e4f-5a6b7c8d9e0c");
-
     /// <summary>A server's id in Spelunk.</summary>
     internal static Guid SpelunkServer { get; } = Guid.Parse("0192f0c8-dddd-7c3d-8e4f-5a6b7c8d9e0d");
 
     /// <summary>
-    /// A route to <see cref="SpelunkServer"/>, with <see cref="SpelunkKey"/> — signing in through Spelunk
-    /// when <paramref name="signIn"/> says so.
+    /// A route to <see cref="SpelunkServer"/> — signing in through Spelunk when <paramref name="signIn"/> says so.
     /// </summary>
     internal static SpelunkRoute ThroughSpelunk(string apiBase = "https://api.spelunk.example.com", bool signIn = false) =>
-        new() { ApiBase = apiBase, ServerId = SpelunkServer, ApiKeyCredentialId = SpelunkKey, SignInThroughSpelunk = signIn };
+        new() { ApiBase = apiBase, ServerId = SpelunkServer, SignInThroughSpelunk = signIn };
 
     /// <remarks>
     /// <paramref name="port"/> defaults to an explicit 22 rather than to null, so a test that says nothing