fix: guard SubagentStop hook to only fire during active workflows

[avifenesh]

Summary

• Switch SubagentStop hook from unconditional prompt type to guarded command type
• Add subagent-stop-guard.js - a Node.js script that checks for active flow.json before injecting the workflow enforcement prompt
• Uses Node instead of bash for cross-platform support (Windows + macOS + Linux)
• Add 12 tests covering no-op cases (no flow, completed, aborted, failed, corrupted JSON) and active workflow cases

Problem

The prompt type SubagentStop hook fired on every subagent stop across the entire session, not just during /next-task workflows. An exploration agent wasted 136K tokens and 19 minutes processing gate logic before returning "All gates blocked."

Fix

The guard script reads {stateDir}/flow.json using the existing platform-aware state directory detection. It only outputs the enforcement prompt to stderr when status === 'in_progress' and phase !== 'complete'. Otherwise it exits silently (no-op).

Closes agent-sh/agentsys#325

Test plan

• 7 no-op tests: no flow.json, completed, aborted, failed, corrupted JSON, empty, phase=complete edge case
• 5 active workflow tests: implementation phase, exploration phase, all sections present, missing task title, missing task object
• Pre-push hooks pass
• Manual: enable plugin, verify non-workflow subagents complete without delay
• Manual: run /next-task workflow, verify enforcement prompt still appears between phases

[gemini-code-assist]

Code Review

This pull request replaces the static subagent-stop hook with a dynamic Node.js guard script, subagent-stop-guard.js, which injects workflow enforcement prompts only when a workflow is actively in progress. It also introduces a comprehensive test suite for the guard script and adds a test execution script to package.json. The review feedback recommends simplifying the state directory path resolution by utilizing existing library helpers and adding a fallback value for the workflow phase in the generated prompt to prevent 'undefined' output.

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: 66951f0517

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".