Feature: Support nested combo script execution

[linw1995]

Overview

Enable combo scripts to invoke coco combo run commands, allowing nested combo execution for modular and reusable automation workflows.

Current State

The codebase currently blocks combo run requests within combo sessions:

• src/combo/starter.rs:1231-1237 rejects ClientMessage::ComboRun with "combo run is not allowed in combo session"
• No recursion depth limit exists
• Each combo creates an independent SessionEnv and socket

Proposed Solution

1. Add Recursion Protection

• Introduce COCO_COMBO_DEPTH environment variable
• Set maximum depth limit (e.g., MAX_COMBO_DEPTH = 5)
• Increment depth on nested calls

2. Remove Blocking Logic

• Modify src/combo/starter.rs to allow ComboRun messages with depth validation
• Return clear error when depth limit exceeded

3. Environment Propagation

• Ensure nested combos inherit appropriate context
• Handle socket path routing for multi-level sessions

Acceptance Criteria

• Combo scripts can successfully execute coco combo run <name> commands
• Recursion depth is tracked via COCO_COMBO_DEPTH env var
• Depth limit (default: 5) prevents infinite recursion
• Clear error message when limit exceeded
• Nested combo events properly route to parent session
• Test coverage for:
  • Single-level nesting
  • Multi-level nesting (3-5 levels)
  • Recursion limit enforcement
  • Error handling

Implementation Notes

Key files to modify:

• src/combo/starter.rs - Remove blocking, add depth check
• src/combo/session_env.rs - Add depth env var
• src/tools/run_combo.rs - Propagate depth context
• src/cmd/combo.rs - CLI entry handling

Technical considerations:

• Each nested combo creates independent SessionEnv (existing behavior)
• Child processes inherit COCO_SESSION_SOCK (existing behavior)
• Event routing complexity increases with nesting levels