feat(settings): default settings.enablePluginPadOptions to true (#7841)

This flag gates the ep_* passthrough on padoptions that shipped in 3.0.0
(PR #7698). It was introduced as opt-in, but the intent in shipping it
was to let plugins like ep_plugin_helpers' padToggle / padSelect ride
the existing broadcast/persist rail out of the box — flipping the
default closes the gap.

Why now
- ep_comments_page#422 (and sibling per-plugin reports discussed on
  Discord): stock 3.x deployments console.warn on every pad load because
  the helper detects clientVars.enablePluginPadOptions === false and
  tells the admin to flip it. With the flag default-true, the warning
  stops firing on fresh installs while still surfacing for operators
  who have explicitly opted out.
- Plugins that already depend on ep_plugin_helpers >= 0.6 expect the
  pad-wide path to work; the default-false gate silently no-op'd
  pad.changePadOption('ep_*', …) and made the helper UI inert.

Scope
- Settings.ts default flipped to true; comment rewritten to describe
  the new "operator opt-out" model rather than the old AGENTS.MD §52
  opt-in framing (that policy still applies to *new* features; this
  one has shipped and proven safe).
- settings.json.template env-var substitution default flipped to true
  so docker / supervisor configs without an explicit value get the
  new behavior.
- doc/plugins.md updated to match (default true, opt-out via
  settings.json) and the PluginCapabilities source comment.
- Backend test describe-blocks relabeled — "true" is now "(default)",
  "false" is now "(operator opt-out)". Both branches still cover the
  same matrix so the size-cap / namespace-validation paths stay
  exercised.

Compat
- Existing deployments with an explicit `"enablePluginPadOptions":
  false` in settings.json keep that value — no migration needed.
- Older clients only read clientVars.enablePluginPadOptions; the
  protocol shape is unchanged.

Closes ep_comments_page#422 (helper warning suppression for stock
deployments).

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

settings.json.template

@@ -853,11 +853,11 @@
     * accepting pad-wide values under plugin-namespaced keys matching
     * /^ep_[a-z0-9_]+$/ (e.g. ep_table_of_contents). Values are validated
     * (JSON-safe, 64 KB per key, 256 KB total) and broadcast to every
-    * connected client just like native pad-wide toggles. Disabled by
-    * default; flip to true once your plugins (e.g. ep_plugin_helpers'
-    * padToggle) require it. See doc/plugins.md.
+    * connected client just like native pad-wide toggles. Enabled by
+    * default; set to false to lock plugins out of pad-wide state. See
+    * doc/plugins.md.
     **/
-    "enablePluginPadOptions": "${ENABLE_PLUGIN_PAD_OPTIONS:false}",
+    "enablePluginPadOptions": "${ENABLE_PLUGIN_PAD_OPTIONS:true}",
 
   /*
    * Optional privacy banner shown once the pad loads. Disabled by default.