v0.2.0

feat: --publish maps extra loopback-only host ports into the instance

An agent could start a dev server inside a sandbox but nothing except the
MCP bridge (and optionally --api-port/--web) reached the host, so the
operator could not open what the agent built. `create --publish` repeats
and takes docker's -p order: bare `5173` reuses the number on the host,
`18090:8090` remaps when the host port is taken.

Host ports are checked against the instance's own MCP/web/api ports and
against the machine before anything is written, so a bad flag leaves no
half-created instance. Pairs persist in .env as PUBLISH=host:container,...
and render as literal 127.0.0.1 entries on the opencode service — compose
cannot expand one variable into a list, so API_PORT keeps its ${} form and
--publish does not.

Closes #2