[FEATURE] Configurable auto-compact threshold (default 50%, disable with 0%)

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

Summary

Currently, auto-compact triggers at 100% context usage with no way to customize the threshold. I'd like to be able to set a custom threshold so that auto-compact fires earlier — before the context window fills up completely.

Proposed Behavior

Add a configurable setting in settings.json:

{
  "autoCompactThreshold": 50
}


### Proposed Solution

## Proposed Behavior

Add a configurable setting in settings.json:

```json
{
  "autoCompactThreshold": 50
}


Range: 0–100 (percentage)
Default: 50 — auto-compact triggers when context reaches 50%
0: disables auto-compact entirely (opt-out for manual /compact users)
100: preserves current behavior (no breaking change for existing users)


Why This Matters
At 100%, auto-compact kicks in unexpectedly in the middle of work, causing context loss without warning
Users who want consistent, predictable summaries prefer compacting earlier (e.g., 30–50%) while the conversation is still coherent
Setting 0 gives power users full manual control via /compact


### Alternative Solutions

None — the threshold is hardcoded and cannot be changed through settings or hooks.

### Priority

Critical - Blocking my work

### Feature Category

Interactive mode (TUI)

### Use Case Example

_No response_

### Additional Context

_No response_