Session hangs after work appears complete; Escape recovery enters permanent "Cancelling"

### Describe the bug

After a turn appears to finish its work, the CLI can hang in a busy/thinking state and never return to a usable prompt. The session is already stuck before any user intervention.

Pressing **Escape** to try to recover from that stuck state makes things worse: the UI transitions into a permanent **`Cancelling`** state. Once there, normal input and slash commands stop working entirely, and the only recovery is killing and restarting the process.

This report covers two compounding failures:

1. **Post-completion hang** — the session does not return to the prompt after work appears complete
2. **Escape recovery failure** — pressing Escape while stuck enters permanent `Cancelling` instead of recovering

### Known observations

- visible work appeared complete, but the CLI did not return to a normal ready prompt
- after the session remained stuck for a while, pressing **Escape** moved the UI into **`Cancelling`**
- once in that state, normal input and slash commands no longer worked
- the session had to be restarted/killed to recover

### Observed sequence

1. Start an interactive Copilot CLI session.
2. In at least one observed case, the session remained busy for a long time.
3. The visible work appeared complete, but the CLI did not return to a normal ready prompt.
4. After waiting and seeing no recovery, press **Escape** to try to recover or cancel the stuck state.
5. The UI switches to **`Cancelling`** and does not recover.
6. Normal input and slash commands no longer work.

**Reproduction status:** Intermittent. No minimal deterministic repro is known yet. The conditions that trigger the initial post-completion hang have not been isolated.

### Expected behavior

When work is complete, the CLI should reliably return to a usable prompt instead of remaining stuck in a busy state.

If **Escape** is pressed after the session is already stuck, it should either:

- cleanly recover or cancel, or
- safely no-op while preserving input

It should not transition into a permanent **`Cancelling`** state that disables further interaction.

### Affected version

GitHub Copilot CLI **1.0.25**

### Environment

- **OS:** Linux

### Potentially relevant context (not confirmed triggers)

- observed in an interactive / non-autopilot session
- the session may have involved background work or sub-agent activity, but that has **not** been isolated as the trigger
- the primary failure appears to be the post-completion stuck state; the failed Escape recovery appears to be a secondary consequence

### Related issues / lineage

- **#405** — strongest older precedent for sessions hanging after task completion
- **#2617** — narrower adjacent post-completion stale-status report that was closed as fixed in v1.0.21
- **#2557** and **#2502** — similar stuck-`Cancelling` outcomes, but during active cancellation rather than after an already-stuck post-completion state
- **#2681** — related Esc/input-state restoration bug on a different path

Issues in this symptom family were closed as fixed through v1.0.21, but this was observed on v1.0.25. The exact relationship is still uncertain.