Add hourly GitHub Actions workflow to auto-fix issues labeled agent_fix

[hahacium]

Motivation

• Automatically detect open issues labeled agent_fix and attempt small, safe repository fixes on an hourly schedule to reduce manual triage overhead.
• Ensure repairs are reproducible, scoped, and reviewed by creating a repair branch and PR instead of pushing direct changes.

Description

• Add a scheduled and manually-triggerable workflow at .github/workflows/agent-fix.yml that selects one open issue with the agent_fix label, skips if a corresponding PR already exists, creates a branch, runs a Codex repair agent, commits repository changes when present, and opens a PR.
• Add a reusable Codex prompt at .github/codex/prompts/agent-fix.md that instructs the repair agent to reproduce the issue when possible, make the smallest safe change, preserve project style, run lightweight checks, and produce a concise final summary.
• The workflow builds a combined prompt file (codex-agent-fix-prompt.md), invokes openai/codex-action@v1 with sandbox: workspace-write, removes temporary prompt/output files before checking for repository changes, and uses printf to safely construct the PR body.
• The job includes concurrency control and checks to avoid duplicate processing and to only commit when there are actual changes.

Testing

• Attempted YAML validation with python3 + yaml.safe_load failed due to the environment missing the PyYAML module (expected failure).
• Validated the workflow YAML with Ruby/Psych using ruby -e 'require "psych"; Psych.load_file(ARGV[0]); puts "#{ARGV[0]}: ok"' .github/workflows/agent-fix.yml, which succeeded.
• Confirmed repository state and staged changes using git status --short and git diff --cached --stat, which showed the two new files were added.

---

Codex Task