Fix auth fallback and shutdown completions

[cbusillo]

Summary

• fall back to cached ChatGPT access tokens when proactive refresh fails but the token is still unexpired
• keep expired tokens failing when refresh cannot complete
• mark replaced/shutting-down sessions inactive so late background completions cannot enqueue hidden follow-up turns

Validation

• cargo test -p code-core proactive_refresh --lib
• cargo test -p code-core auth_for_stored_account --lib
• cargo test -p code-core agent_completion_wake --lib
• ./build-fast.sh