docs: Document agent container build/test environment from epic #11970

[Copilot]

Epic #11970 introduced agent container parity with GitHub Actions runners through PRs #11979, #11980, and #12062. The existing sandbox.md documentation was outdated—showing only 3 mounted utilities when 20 are now available.

Changes to docs/src/content/docs/reference/sandbox.md

• Default Mounted Volumes: Added missing mounts (${HOME}/.cache, /opt/hostedtoolcache, /opt/gh-aw)
• Mounted System Utilities: New section documenting all 20 utilities
  • Essential (8): cat, curl, date, find, gh, grep, jq, yq
  • Common (12): cp, cut, diff, head, ls, mkdir, rm, sed, sort, tail, wc, which
• Runtime Tools (hostedtoolcache): New section covering:
  • Available runtimes (Node.js, Python, Go, Ruby, Java)
  • PATH integration via find /opt/hostedtoolcache -maxdepth 4 -type d -name bin | tr '\n' ':'
  • Version priority for actions/setup-* configured versions

Example workflow pattern

jobs:
  setup:
    steps:
      - uses: actions/setup-go@v5
        with:
          go-version: '1.25'
      - uses: actions/setup-python@v5
        with:
          python-version: '3.12'
---

Use `go build` or `python3` in your workflow - both are available!

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• telemetry.astro.build
  • Triggering command: /opt/hostedtoolcache/node/24.13.0/x64/bin/node node /home/REDACTED/work/gh-aw/gh-aw/docs/node_modules/.bin/astro build (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

Original prompt

Find all the PRs merged into main that's related to this epic: #11970. Understand the design approach, and improve current documentation so that people can understand this feature

Custom agent used: technical-doc-writer
AI technical documentation writer for GitHub Actions library using GitHub Docs voice

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