Opus 4.7 small context window triggers auto-compact too frequently

### Describe the bug

When using `Opus 4.7` with a Copilot Pro+ subscription, the effective available context window appears much smaller than comparable models like `GPT 5.4` under the same conditions.

In practice, this causes auto-compact to trigger very frequently, including multiple times within a single prompt/session, which makes the model difficult to use even for medium-complexity tasks.

This is especially noticeable because Opus-class models are generally better suited for more complex tasks, which often require larger working context. I understand that the 1M tokens context allocation may not be feasible for cost or product reasons, but the current limit seems too restrictive to be practical.

In my case, a large portion of the context appears to be consumed by `System/Tools`, leaving much less room for actual task context than with `GPT 5.4`.

`Opus 4.7` context

`GPT 5.4` context (for comparison using the same plugins and tools on the same machine)

### Affected version

GitHub Copilot CLI 1.0.36

### Steps to reproduce the behavior

1. Start a fresh Copilot CLI session on a Pro+ subscription.
2. Switch to `Opus 4.7` model.
3. Run a simple prompt to populate the context and observe that a large share of the context is already occupied by `System/Tools`.
4. In the same setup, read a moderate amount of content (for example, around 10 markdown files of roughly 200-300 lines / 15-20 KB each) and ask for a summary (or any other medium effort task).
5. Observe that the context fills up and auto-compact triggers quickly, often multiple times during the same session or even during a single prompt.
6. Repeat the same workflow with `GPT 5.4` (Or any other comparable model like Sonnet) and compare effective remaining context and auto-compact frequency.

### Expected behavior

1. `Opus 4.7` should reserve a smaller portion of the window for `System/Tools`, closer to `GPT 5.4` (Or any other comparable model like Sonnet) under the same setup.
2. A medium-complexity prompt should fit without repeated auto-compact during normal use.
3. If the model must have a smaller context budget than GPT 5.4, it should still be large enough to handle moderate multi-file workflows reliably.

### Additional context

- OS: Windows 11
- Shell: PowerShell
- Subscription: Copilot Pro+
- Comparison was made using the same plugins/tools setup as `GPT 5.4`

Non viene indicato alcun file sorgente, test o punto di ingresso. Inizia riproducendo il confronto in Copilot CLI 1.0.36 con Opus 4.7 e GPT 5.4 usando gli stessi plugin e strumenti, quindi traccia come vengono selezionati l’utilizzo del contesto System/Tools e le soglie di auto-compact. Il lavoro è completato quando i prompt medi su più file non attivano più auto-compaction ripetute, oppure quando la limitazione specifica del modello viene gestita chiaramente.

Scritto dal modello di indicizzazione a partire dal testo della issue.