fix(cli): user-initiated abort no longer kills the ACP runner process

[FelixShowX]

Problem

When the user taps "stop" in the mobile app during an ACP agent turn (gemini / opencode / any generic ACP backend), the CLI process crashes and the whole session dies, instead of just cancelling the in-flight turn.

Root cause

AcpBackend.cancel() emits status: 'stopped' after a successful session/cancel:

// AcpBackend.ts
async cancel(sessionId: SessionId): Promise<void> {
  ...
  await this.connection.cancel({ sessionId: this.acpSessionId });
  this.emit({ type: 'status', status: 'stopped', detail: 'Cancelled by user' });
}

But runAcp treats any stopped status as fatal backend death:

if (msg.status === 'error' || msg.status === 'stopped') {
  stopRunnerFromBackendStatus(msg.status, msg.detail); // → shouldExit + reject pending turn → process exits
}

So a perfectly healthy user abort is indistinguishable from the backend dying, and the runner shuts the session down. The turn IS cancelled correctly on the agent side — the runner just kills itself right after.

Fix

Track user-initiated aborts and treat the resulting stopped as a normal (cancelled) turn end:

• handleAbort() sets userAbortInFlight = true before calling backend.cancel()
• When a stopped status arrives with that flag set: reset thinking/keepAlive, resolve the pending turn (instead of rejecting it), and mark the turn cancelled — the runner stays alive and the next prompt works
• The flag is cleared at every new turn start, and also if cancel() throws (the abort never reached the agent). An abort that arrives while the session is idle — e.g. network latency delivers it right after the turn ended naturally — would otherwise leave the flag set, and a later genuine backend failure would be misread as a user abort, leaving the runner alive with a dead backend. Bounding the flag's effect to its own turn prevents that
• turn-end is reported as 'cancelled' instead of 'completed'

Repro

1. happy acp <agent> (any ACP agent)
2. From the phone app, send a long-running prompt (e.g. "count from 1 to 1000, one per line")
3. While it streams, tap stop
4. Before: CLI process exits with Error: <agent> backend stopped: Cancelled by user. After: turn ends as cancelled, session stays online, next prompt works.

Notes

• No behavior change for genuine backend failures: a stopped status without a preceding user abort (same-turn) still shuts the runner down
• AcpSessionManager.endTurn already accepts 'cancelled' — no other files touched
• Verified against kimi-code 0.34.0 via the ACP protocol; patch compiles and passes the full upstream unit suite (792/792) on a clean upstream/main checkout