feat: support custom container environment variables

[Alice39s]

Summary

• add repeatable -e/--env KEY=VALUE options for container sessions
• support project-level defaults through agent.environment in .agentc/settings.json
• merge project and CLI environment variables by name, with CLI values taking precedence
• persist environment options when generating settings with agentc init

Motivation

Container images don't always ship sane locale and time-zone defaults, so tools can behave differently across hosts. Setting variables like TZ, LANG, and LC_ALL explicitly fixes that without having to build a custom image.

Usage

Pass one or more variables on the command line:

agentc run \
  --env TZ=America/Los_Angeles \
  --env LANG=en_US.UTF-8 \
  --env LC_ALL=en_US.UTF-8

Or define project defaults in .agentc/settings.json:

{
  "agent": {
    "environment": {
      "TZ": "America/Los_Angeles",
      "LANG": "en_US.UTF-8",
      "LC_ALL": "en_US.UTF-8"
    }
  }
}

CLI values override project settings with the same name:

agentc run --env TZ=Europe/Berlin

Empty values and values containing additional = characters are preserved. If a CLI variable is repeated, the last value wins.

[laosb]

Thank you!