fix(agents): validate sandbox names before Gator launch setup

[TaylorMutch]

Summary

Following the Gator launch skill with gator-pr-4633-supervised built the image and configured providers before the gateway rejected the 24-character name. Use names within the gateway's 19-character DNS-label limit and reject invalid names before launch setup.

Related Issue

Closes #4659

Author access verified as ADMIN; the issue does not have state:accepted.

Changes

• Shorten PR, issue, and experimental watcher names throughout the launch skill and tighten its validation to 19 characters.
• Validate explicit, environment-provided, and generated sandbox names before image builds, credential discovery, or gateway mutations.
• Document the name limit and the generated-name prefix budget.

Testing

• Checks appropriate to the affected code and behavior pass: Bash syntax, changed Markdown/skill lint, skill example validation, and git diff --check.
• Unit tests added/updated (not applicable).
• E2E tests added/updated (not applicable: host-side launcher validation; no sandbox runtime or deployment changes).

ShellCheck reports no errors in the launcher. Existing launcher warnings remain in unchanged image-build code. The original failure and a successful launch with gtr-pr-4633-watch were observed on a local Podman gateway before this change. Invalid names were also checked locally during development; no test script is included.

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (not applicable)