feat(skills): add cross-machine-coordination skill for multi-machine squad deployments

[tamirdresher]

Summary

Adds a new cross-machine-coordination skill that enables squad agents running on different physical machines to coordinate work without human intervention.

Problem

In multi-machine squad deployments (e.g., laptop + DevBox + Azure VM), agents on different machines need to:

• Assign work to machines with specific capabilities (GPU, high memory, etc.)
• Pass task results back to the originating machine
• Handle urgent ad-hoc tasks across machines

Without this, humans must manually copy files, paste commands, and relay results between machines — defeating the purpose of a autonomous agent squad.

Solution

This skill defines a git-based task queue protocol:

1. Orchestrating machine writes a YAML task file to .squad/cross-machine/tasks/
2. Commits and pushes to the shared git repo
3. Executing machine's Ralph polls on each watch cycle, validates, and executes
4. Result YAML written to .squad/cross-machine/results/, committed and pushed
5. Originating Ralph picks up the result on next cycle

For urgent tasks: GitHub Issues with squad:machine-{name} labels provide an alternative channel that Ralph monitors.

Skill Contents

• Task file format (YAML schema with required/optional fields)
• Result file format (status, exit code, stdout/stderr, artifacts)
• Security validation pipeline (schema validation, command whitelist, resource limits, execution isolation, audit trail)
• Ralph Watch integration (automatic poll-and-execute loop)
• Error handling (failures, timeouts, network interruptions)
• Configuration (per-machine settings in .squad/config.json)
• Full worked examples (GPU workload, urgent debug request)

Origin

Developed and battle-tested in a squad deployment spanning laptop ↔ DevBox ↔ Azure VM workflows. Contributed back upstream as part of the skill library expansion.

Checklist

• Follows existing SKILL.md format
• No personal/environment-specific identifiers
• Security model documented
• Examples included
• Compatible with Ralph Watch loop

[bradygaster]

Hey @tamirdresher 👋 — the CI failures on this PR are from the old dev base (cspell words and a broken link that have since been fixed). The test timeout on platform-adapter.test.ts also appears to be a pre-existing flaky test. If you rebase on current dev, this should go green. Great skill contribution!