chore: drop the legacy youtube block from the secrets template

No code reads a top-level youtube secrets key: the youtube-stats plugin
receives its API key namespaced under its own plugin id (declared via
x-secret in its config schema), like every other store plugin. The key
survives only in state_reconciliation.py's non-plugin-key exclusion set,
which stays — existing installs still carry the key in their generated
config_secrets.json, and the exclusion prevents it from being
misclassified as a plugin config. New installs simply stop being asked
for a YouTube API key they have nowhere to use.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SXb4mKcAkVaxkeTb3YnAdr

config/config_secrets.template.json

@@ -1,8 +1,4 @@
 {
-    "youtube": {
-        "api_key": "YOUR_YOUTUBE_API_KEY",
-        "channel_id": "YOUR_YOUTUBE_CHANNEL_ID"
-    },
     "github": {
         "api_token": "YOUR_GITHUB_PERSONAL_ACCESS_TOKEN"
     }