docs: make ax-cli skill shareable

[madtank]

Summary

• Replace the repo skill with a sanitized, shareable ax-cli skill
• Keep the useful operating model: identity checks, user PAT to agent PAT bootstrap, profiles, send/handoff/task/context workflows, MCP/channel runtime setup, and completion verification
• Remove local/private details: concrete platform URLs, local machine paths, named people/agents, tenant IDs, and internal deployment commands
• Link deeper workflows to existing repo docs instead of embedding private runbook specifics

Validation

• rg check for known private names/URLs/paths/IDs: no hits
• git diff --check: passed
• Pre-commit hooks during commit/push: ruff skipped for docs-only change