feat(opencode): add context overflow prevention with configurable thresholds

[Zena-park]

Fixes #8089, #8293, #6286, #6068 Closes #8140

What does this PR do?

Problem

Auto-compaction was not triggering before context limits were exceeded, causing context_length_exceeded API errors. Users had no control over when compaction triggered or ability to limit context usage for cost optimization.

Solution

#8140 - Configurable context limit and auto-compaction threshold

• Add compaction.threshold config option (0.5-0.99, default 0.9)
• Add compaction.maxContext config option to override model's context limit
• Implement shouldCompact() function in compaction.ts with threshold logic

#8089, #8293, #6286, #6068 - Prevent context_length_exceeded errors

• Pre-check in prompt.ts: estimate tokens before API call, trigger compaction if threshold exceeded
• Post-check in processor.ts: monitor context after tool execution
• Model-adaptive tool output limits in truncation.ts (5% of context, min 10KB, max 2MB)
• Update bash.ts, tool.ts, registry.ts to use dynamic limits based on model size

Config Example

{
  "compaction": {
    "threshold": 0.8,
    "maxContext": 100000
  }
}

How did you verify your code works?

• ✅ 756 tests passing (+8 from baseline)
• ✅ Added 22 new tests (truncation: 14, compaction: 8)
• ✅ Type check passing
• ✅ Verified pre-check triggers compaction before API calls
• ✅ Verified post-check monitors context after tool execution