feat: add SSH plugin with pre_entrypoint and ports support

[dorey-agent]

Summary

Adds an SSH plugin for v1 architecture, enabling remote development access (IDE, debugging, file transfer) inside agent containers.

Plugin Model Extensions

Two new fields added to RuntimeContrib:

Field	Purpose
pre_entrypoint	Commands injected into entrypoint.sh before exec "$@"
ports	Port mappings exposed on the agent container

These are generic enough to support future plugins (VNC, dev servers, etc).

SSH Plugin (core/plugins/ssh/)

• Installs openssh-server at build time
• Starts sshd as a background daemon before the agent CMD
• Key-only auth (password disabled)
• Host key generated at build time (persistent key via runtime.volumes if needed)

Usage

installations:
  - plugin: ssh
    options:
      port: 2222
      authorized_keys: "./ssh_key.pub"

Related

Inspired by PR #89 (v0 SSH support). This is the v1 data-driven equivalent — no Go plugin code needed.

Testing

• Unit tests for EntrypointScript with/without pre_entrypoint
• Unit tests for compose port exposure
• Unit tests for template rendering of new fields
• Unit tests for merge logic
• Full test suite passes, build succeeds