fix(s16): resume teammate after plan approval

[ivaris03]

Summary

• wake an idle teammate when plan_approval_response is received
• keep each idle inbox batch local so previously consumed messages are not re-injected
• start a fresh LLM turn after idle inbox activity instead of appending an empty tool-result user message
• add a deterministic regression test for approval wake-up, context injection, and graceful shutdown

Root cause

The idle loop handled plan_approval_response but did not treat it as a resume signal. When another inbox message did break the idle loop, execution fell through to the tool-result path for a response with no tool calls, appending an empty user content list. The next API call could fail and the teammate thread would exit because that exception is swallowed by the teaching harness.

Validation

• python -m pytest -q tests/test_s16_team_protocols.py
• python -m pytest -q (25 passed, 30 subtests passed)
• python -m py_compile s16_team_protocols/code.py tests/test_s16_team_protocols.py
• git diff --check

Closes #455