feat(telegram,discord): migrate to plugin userConfig secrets

Closes H1 #3617646 via the proper path — plugin userConfig with
sensitive: true routes tokens to keychain (macOS) or .credentials.json
0600 (elsewhere) instead of world-readable .env files.

Requires claude-cli-internal#23383 (PLUGIN_OPTIONS ungate + per-server
sensitive split).

Changes per plugin:
  - plugin.json: add userConfig.{PLATFORM}_BOT_TOKEN with sensitive: true
  - .mcp.json: add env block with ${user_config.{PLATFORM}_BOT_TOKEN}
  - server.ts: update comment + error message to point to
    /plugin reconfigure instead of .env file

The .env read loop stays as a legacy fallback for existing users —
process.env wins (injected value takes precedence), so no migration
forced. New users get prompted at enable time via CC's built-in dialog;
token lands in keychain, never touches settings.json.

/telegram:configure and /discord:configure skills are NOT removed in this
PR — they still work for the legacy .env path. Follow-up: repurpose or
remove after a grace period once cli#23383 is released.

🏠 Remote-Dev: homespace

external_plugins/discord/.claude-plugin/plugin.json

@@ -1,11 +1,20 @@
 {
   "name": "discord",
-  "description": "Discord channel for Claude Code \u2014 messaging bridge with built-in access control. Manage pairing, allowlists, and policy via /discord:access.",
-  "version": "0.0.1",
+  "description": "Discord channel for Claude Code — messaging bridge with built-in access control. Manage pairing, allowlists, and policy via /discord:access.",
+  "version": "0.0.2",
   "keywords": [
     "discord",
     "messaging",
     "channel",
     "mcp"
-  ]
+  ],
+  "userConfig": {
+    "DISCORD_BOT_TOKEN": {
+      "type": "string",
+      "title": "Bot Token",
+      "description": "Bot token from the Discord Developer Portal. Stored in keychain (macOS) or ~/.claude/.credentials.json with 0600 permissions elsewhere. Never written to settings.json.",
+      "required": true,
+      "sensitive": true
+    }
+  }
 }