docs: warn that allowed_bots can expose the action to external triggers (#1039)

allowed_bots does not verify that a matching bot is installed on the
repository or has write access. On a public repo, external GitHub Apps
may be able to trigger workflow events (issues, comments, PR reviews).
If the workflow listens on those events and allowed_bots is '*', an
external App can invoke this action with a prompt it controls.

Default config (allowed_bots: "") is unaffected.

- docs/security.md: add warning and mitigation guidance
- docs/usage.md: add inline warning to the allowed_bots input row
- action.yml: add warning to the allowed_bots input description

🏠 Remote-Dev: homespace

action.yml

@@ -28,7 +28,7 @@ inputs:
     required: false
     default: ""
   allowed_bots:
-    description: "Comma-separated list of allowed bot usernames, or '*' to allow all bots. Empty string (default) allows no bots."
+    description: "Comma-separated list of allowed bot usernames, or '*' to allow all bots. Empty string (default) allows no bots. WARNING: On public repos with '*', external Apps may be able to invoke this action with prompts they control. See docs/security.md."
     required: false
     default: ""
   allowed_non_write_users: