Project sessions hide context-exhausted and interrupted turn causes

[scotttesler]

Describe the bug

A coordinated Copilot project session that exhausts its model context, or whose active resume turn is interrupted by a new parent message, surfaces only a generic error/idle state to the parent. The underlying worktree and commit remain intact, but the parent cannot see whether the cause was context exhaustion, user interruption, or a model/network failure and may incorrectly abandon a safely resumable session.

Affected version

1.0.71-2

Steps to reproduce the behavior

1. Start a coordinated project session with a long task that performs substantial review and validation work.
2. Let the turn reach its context limit after it has committed work but before it reports completion, or send a new parent message while a resumed turn is actively fetching/rebasing.
3. Observe the parent-facing session status and metadata.
4. Inspect the child directly and confirm its worktree/commit are intact and that the actual cause was context exhaustion or interruption.

The parent receives a generic failure state without the actionable cause. In one reproduction, transient model 408 errors also appeared around the interrupted turn, further obscuring the primary cause.

Expected behavior

Parent-facing project-session metadata should expose a safe, structured terminal reason such as context_exhausted, interrupted_by_user, or model_timeout, while clearly indicating that the worktree is intact and the session is resumable. A completed child diagnosis should not be lost at the delivery/UI boundary.

Additional context

• macOS / Apple Silicon
• The committed code was not corrupted or lost.
• A fresh session could recover the commit, but that workaround consumes a new worktree and requires manual diagnosis.
• Please route this to the app/orchestration owner if the status translation is outside the CLI process.