feat(agents): publish payloads for remote gateways

[drew]

Summary

Add an agent-agnostic OCI publisher and integrate it with the manifest-driven
agent launcher so immutable agent payloads work on remote gateways. Remote
launches publish the staged payload first and create the sandbox from the
registry-reported digest instead of a mutable tag.

Related Issue

Closes #2500

Changes

• Add scripts/agents/publish.sh for publishing any Dockerfile/context and
  returning a digest-pinned OCI reference.
• Add --publish-to and --platform launcher options plus environment
  equivalents.
• Detect remote gateways and provide actionable guidance when a local source is
  used without publication.
• Publish before gateway settings or provider mutations.
• Add standalone publisher and full launcher integration tests to mise run test.
• Document the generic publisher, remote gator workflow, security boundary, and
  operator skill guidance.

Testing

• mise run pre-commit passes
• Unit tests added/updated
• E2E tests added/updated (if applicable)

Manual remote validation used PR #2253: the staged gator payload was built for
linux/amd64, pushed to a private GCP Artifact Registry, accepted by
drew-sandbox, and reached Ready as gator-pr-2253-supervised.

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (if applicable)