bug: background-process history evicts still-running entries (insertion-order FIFO ignores status)

## What happened?

Background-process history is capped with a plain insertion-order FIFO: when full, the *oldest inserted* entry is deleted regardless of whether that process is still running. A session that backgrounds many long-lived processes can evict the record of a **live** process; its exit handler then finds no history item and no-ops, so `listBackgroundProcesses()` permanently loses a running process and its exit status is never recorded.

## Affected code

`packages/core/src/services/shellExecutionService.ts:1501-1506`:

```ts
if (history.size >= MAX_BACKGROUND_PROCESS_HISTORY_SIZE) {
const oldestPid = history.keys().next().value;
if (oldestPid !== undefined) {
history.delete(oldestPid);
}
}
```

`Map` iteration is insertion order; nothing checks the evicted entry's `status`. The exit path looks the item up by pid (~lines 796-803 / 1280-1286) and silently does nothing when missing.

## How can this be reproduced?

1. Background `MAX_BACKGROUND_PROCESS_HISTORY_SIZE` long-running commands, keeping the earliest alive.
2. Background one more command.
3. The first (still-running) entry disappears from `listBackgroundProcesses`; when it eventually exits, no completion is recorded.

## What did you expect to happen?

Eviction should prefer completed/failed entries; running processes should never be evicted while alive.

## Suggested direction

Scan for a terminal-status entry to evict first (`history.values().find(v => v.status !== 'running')`) and only fall back to oldest-insertion if all entries are live (or raise the cap).

---

*Found by source audit on current `main` (commit `5411f113c`); platform-independent. No open issue/PR covering this was found (searched: background process list eviction).*

Start in packages/core/src/services/shellExecutionService.ts at the history-cap logic around lines 1501-1506, then read the exit paths around lines 796-803 and 1280-1286. Verify that eviction selects a terminal-status entry before a running one, and reproduce the described sequence to confirm live processes remain listed and later record their exit status.

Written by the indexing model from the issue text.