feat: built-in large tool output handling

[yunus25jmi1]

Summary

Add a new builtin hook that automatically handles large tool responses. When enabled, tool responses exceeding the threshold are saved to disk and replaced with a pointer that the agent can read back using shell tools.

This is a root-cause fix for issue #2722.

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Problem

When an MCP tool returns a large response the agent breaks in one of two ways:

1. With default max_old_tool_call_tokens budget: A single large result eats the entire budget and all earlier tool results in the session get silently replaced with [content truncated].

2. With max_old_tool_call_tokens: -1: The full response stays in the session but overflows the model's context window. Auto-compaction can't help because the oversized result is in the most recent messages, so the agent gets stuck.

Worth noting: builtin tools (shell, openapi, api) each have a hardcoded 30,000-char per-result limit. MCP tools have no per-result limit at all.

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Root Cause

MCP tools have no output limit, unlike builtin tools (shell, openapi, api) which all have a hardcoded 30,000-char limit. When MCP returns large responses, they flow into the session unmodified.

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Solution

Added a new builtin hook handle_large_tool_output that:

1. Intercepts tool responses at EventToolResponseTransform
2. When response exceeds threshold, saves full output to disk
3. Replaces response with a pointer showing tool name, char count, file path, preview

Configuration

agents:
  root:
    handle_large_tool_output:
      enabled: true        # default: false
      threshold: 5000      # chars - default
      output_dir: /tmp     # default: os.TempDir()
      preview_size: 3000   # chars in preview - default

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Files Changed

File	Change
agent-schema.json	Added handle_large_tool_output config
pkg/config/latest/types.go	Added HandleLargeToolOutputConfig struct
pkg/agent/agent.go	Added field and accessor
pkg/agent/opts.go	Added WithHandleLargeToolOutput option
pkg/teamloader/teamloader.go	Wired config to agent
pkg/hooks/builtins/handle_large_tool_output.go	New builtin hook
pkg/hooks/builtins/builtins.go	Registered builtin
pkg/runtime/hooks.go	Passed config to ApplyAgentDefaults

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Testing

• Build: ✅ Passes
• Lint: ✅ 0 issues
• Docker Image: ✅ Builds

Fixes #2722