feat: gate legacy startup sidecar fallback

docs/plugins/compatibility.md

@@ -136,7 +136,8 @@ Current compatibility records include:
   move to `openclaw/plugin-sdk/channel-route`
 - activation hints that are being replaced by manifest contribution ownership
 - deprecated implicit startup sidecar loading for plugins that have not declared
-  `activation.onStartup`
+  `activation.onStartup`; maintainers can test the future stricter behavior with
+  `OPENCLAW_DISABLE_LEGACY_IMPLICIT_STARTUP_SIDECARS=1`
 - `setup-api` runtime fallback while setup descriptors move to cold
   `setup.requiresRuntime: false` metadata
 - provider `discovery` hooks while provider catalog hooks move to

docs/plugins/manifest.md

@@ -268,6 +268,12 @@ plugins unless they declare `activation.onStartup: true`. Plugin status and
 compatibility reports warn with `legacy-implicit-startup-sidecar` when a plugin
 still relies on that fallback.
 
+For migration testing, set
+`OPENCLAW_DISABLE_LEGACY_IMPLICIT_STARTUP_SIDECARS=1` to disable only that
+deprecated fallback. This opt-in mode does not block explicit
+`activation.onStartup: true` plugins or plugins loaded by channel, config,
+agent-harness, memory, or other narrower activation triggers.
+
 ```json
 {
   "activation": {