Batch API missing oneshot + plugin allowlist validation checks

[staging-devin-ai-integration]

Problem

handle_apply_batch in apps/skit/src/websocket_handlers.rs does not enforce the same oneshot-only node rejection (streamkit::http_input, streamkit::http_output) or plugin allowlist (perms.is_plugin_allowed) checks that handle_add_node does (lines 432-453).

This means a user could:

1. Add oneshot-only marker nodes via the batch API
2. Bypass the plugin allowlist via the batch API

Location

• handle_add_node: apps/skit/src/websocket_handlers.rs:432-453 (has the checks)
• handle_apply_batch: apps/skit/src/websocket_handlers.rs:1259-1320 (missing the checks)
• handle_validate_batch: apps/skit/src/websocket_handlers.rs:1135-1191 (also missing)

Suggested fix

Add the same oneshot and plugin allowlist checks to the batch validation loop. The permission loop at lines 1259-1320 already validates perms.is_node_allowed(kind) and file security — the oneshot and plugin checks should be added alongside them.

Context

Identified during review of PR #286 (async AddNode). This is a pre-existing issue, not introduced by that PR.