M21-P3: Aggressive context pruning in tool loops

[bug-ops]

Parent: #336

Summary

Reduce history tokens sent per tool loop iteration by pruning tool outputs inline and limiting retained results.

Requirements

1. After each tool result in native tool loop, immediately prune outputs older than last 2 iterations
2. Keep only last N tool results in full detail, replace older ones with one-line summaries
3. Configurable max_tool_iterations with sensible default (e.g., 15)
4. Account for tool result tokens separately in budget allocation

Acceptance Criteria

• Tool outputs beyond retention window are pruned inline
• Total context growth is sub-linear with tool iterations
• Default max_tool_iterations prevents runaway consumption

Files

• crates/zeph-core/src/agent/streaming.rs
• crates/zeph-core/src/agent/context.rs