feat: Cross-platform bootstrap for the ops starter agent (Linux / macOS / cloud VM & Docker host)

[vybe]

Summary

Make the ops/DevOps starter agent easy to start on the platforms operators actually run Trinity on — self-hosted Linux servers, macOS dev hosts, and cloud VMs / Docker hosts — with a near-one-command bootstrap per platform so "all sorts of different platforms" are covered out of the box.

Context

Trinity is sovereign infrastructure deployed on heterogeneous hardware. An onboarding starter only lowers time-to-value if it starts cleanly wherever the operator runs Trinity, without per-platform yak-shaving (Docker presence, socket access, ports, credentials).

Acceptance Criteria

• Documented, near-one-command bootstrap for: self-hosted Linux server, macOS, and a generic cloud VM / Docker host
• Each path verifies prerequisites (Docker running, required ports, MCP connectivity) and fails with actionable messages
• Platform differences (paths, socket/group access, ports) are handled or clearly documented
• The same starter agent definition works across all supported platforms (no per-platform forks of the agent)

Technical Notes

• /trinity:deploy-new-instance already handles fresh installs and existing instances on a server — extend/reuse rather than reimplement.
• Mind container-security invariants (non-root, group_add: ${DOCKER_GID} on Linux for Docker socket access) when documenting host setup.

Parent epic: ops/DevOps starter agents for one-click instance management.