fix(code): register task completion tool

[Anduin9527]

Summary

• register SubmitTaskCompleteHandler in the shared Web/headless Code tool registry
• pin the terminal tool in the production-registry ACL test

The task executor already advertises submit_task_complete and uses it as the terminal handshake. Without this registration, otherwise successful implementation tasks fail before dispatch and continue consuming model turns.

Fixes #458

Tests

• cargo +nightly fmt --all --check
• focused container test code_tool_acl_test::dev_context_filter_keeps_all_registered_tools (1 passed)