[FEATURE]: expose more v2 session APIs to plugins #47229

Feature hasn't been suggested before.

• I checked existing issues first. v2: internal tool plugins expose gaps in the public plugin API #34957 is related, but broader. This issue is only about exposing existing session APIs to external v2 plugins.

This recreates #35443, which was automatically closed after 60 days of inactivity.

Describe the enhancement you want to request

While porting a plugin from the v1 plugin API to the v2 plugin API, I ran into a small gap in the public session runtime.

The v2 plugin context already covers prompt, command, interrupt, wait, and compact (compact landed in #52385). Two session APIs that already exist elsewhere in opencode are still not available to plugins:

context.session.shell(...)
context.session.active(...)

These are useful for plugins that coordinate work around the current session. For example, a queue plugin may need to run a shell command in session context, or check if the current run is still active before sending the next item (wait covers the blocking case, active covers the pollable check).