Add configurable auto-compaction threshold to config.json

### Describe the feature or problem you'd like to solve

When using slower, higher-capacity models like Claude Opus 4.6, context window bloat significantly degrades performance well before the CLI's built-in compaction triggers. At ~45-60% context usage: - Response latency increases dramatically (40-90s+ per turn) - Tool call quality degrades — the model makes redundant/malformed calls and repeats work - Rate limit errors become frequent due to higher per-request compute cost - Compaction itself becomes slow because it has to summarize a larger message history In practice, the optimal time to compact with Opus is around 30-35% usage, but the CLI doesn't offer any way to configure this.

### Proposed solution

Add an auto_compact_threshold field to ~/.copilot/config.json:

{
"auto_compact_threshold": 0.30
}

Why this matters

- The current implicit threshold is tuned for faster models (Sonnet/Haiku) where high context usage is tolerable.
It doesn't account for the disproportionate latency penalty on slower models.
- Users who compact proactively at 30-35% see significantly better results: faster responses, fewer errors, better
tool call efficiency, and fewer rate limit hits.
- Power users currently have no mechanism to automate this — it requires manually running /context and /compact
every few turns.

Additional Considerations

- Could support per-model thresholds: "auto_compact_threshold": { "default":
0.50, "claude-opus-4.6-fast": 0.30 }
- Could support a "auto_compact_mode": "auto" | "prompt" to let users choose between silent compaction vs. a
confirmation prompt
- Reasonable default could remain at current behavior (~70-80%) so existing users aren't affected

### Example prompts or workflows

Example Workflows

1. Long refactoring session with Opus A developer is refactoring an auth module across 15 files using Opus 4.6 for
its superior reasoning. After 8-10 turns, context hits 40% and responses slow to 60s+. They don't notice until
they're at 55% and the model starts re-reading files it already processed. With "auto_compact_threshold": 0.30,
compaction fires early while it's still fast (~20s), and the session stays responsive throughout.

2. Per-model tuning for mixed-model workflow A user switches between Sonnet (fast, tolerates high context) and Opus
(slow, needs lean context). With per-model thresholds, they configure { "default": 0.50, "claude-opus-4.6-fast":
0.30 } and get optimal behavior regardless of which model they're using — no manual intervention needed.

3. Autopilot mode without babysitting In experimental autopilot mode, the agent chains actions without user
confirmation. Without auto-compaction, the context silently bloats until the agent degrades — making duplicate tool
calls, generating malformed JSON, and burning through rate limits. A 30% threshold keeps autopilot running cleanly
without the user needing to interrupt with /compact.

4. Rate limit avoidance during crunch time A developer on a deadline keeps hitting "rate limit, try again in 1
minute" errors. They don't realize that their 120k-token context is causing each request to consume disproportionate
compute. Auto-compaction at 30% would keep requests lightweight, reducing rate limit pressure and eliminating forced
cooldown periods.

5. Team standardization via repo-level config A team adds "auto_compact_threshold": 0.35 to their shared config so
that all developers using Copilot CLI on their repo get consistent, performant behavior — especially onboarding
developers who don't yet know to manually compact.

### Additional context

_No response_