fix(agent-teams): resume owned in-progress tasks from IDLE

[hichipli]

Summary

• let an IDLE teammate resume its own unfinished in_progress task befor looking for new work
• reuse the existing assignment/worktree recovery path for resumed work
• apply the fix to the current Agent Teams lesson and the integrated harness
• add one regression test that runs against both lesson implementations
• keep the English, Chinese, and Japanese lesson documentation synchronized

Context

The lesson numbers have changed since #519 was filed. The affected current implementations are s13_agent_teams and s15_integrated_harness.

Previously, a teammate that ended a model turn without calling complete_task entered IDLE with an owned in_progress task. IDLE only attempted to claim new work, while the task ownership invariant correctly prevented a second claim. The teammate could therefore loop forever without resuming the task it already owned.

The updated order is:

1. process mailbox messages
2. resume the teammate's unfinished assignment
3. atomically claim a new ready task only when no owned task exists

Validation

• python3 -m unittest tests.test_agent_teams_runtime.AgentTeamsRuntimeTests.test_idle_teammate_resumes_owned_task_before_claiming_new_work
• python3 -m unittest tests.test_agent_teams_runtime
• 65 runtime tests passed
• python3 -m py_compile s13_agent_teams/code.py s15_integrated_harness/code.py tests/test_agent_teams_runtime.py
• npm run extract from web/
• the regression test covers both s13_agent_teams and s15_integrated_harness
• git diff --check
• the Python examples in the English, Chinese, and Japanese S13 READMEs are byte-for-byte identical

Closes #519.