Ignore transient tool idle boundaries during Loop runs

[ByBrawe]

Fixes #160.

OpenCode 1.x may emit session.idle between tool steps while the same assistant turn is still in progress. The scheduler treated that boundary as final, which could clear the active Loop run and admit another prompt after a shell command.

This change keeps the existing idle fallback when message completion evidence is unavailable, but fails closed when the host message API positively reports the current assistant turn as incomplete.

Regression coverage verifies that:

• unknown completion evidence preserves the historical idle fallback;
• an incomplete assistant turn cannot finalize the active Loop run;
• a completed assistant turn still finalizes normally;
• active tool calls remain a hard busy guard.

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.