Implement OpenCode fork support

[moonray]

This is a follow-up to #148

OpenCode 1.2.24 now allows you to fork a session using opencode --continue --fork or opencode --session [session id] --fork
Please enable the feature in Agent Deck ui to fork session for OpenCode.

Commands:
  opencode completion          generate shell completion script
  opencode acp                 start ACP (Agent Client Protocol) server
  opencode mcp                 manage MCP (Model Context Protocol) servers
  opencode [project]           start opencode tui                                          [default]
  opencode attach <url>        attach to a running opencode server
  opencode run [message..]     run opencode with a message
  opencode debug               debugging and troubleshooting tools
  opencode auth                manage credentials
  opencode agent               manage agents
  opencode upgrade [target]    upgrade opencode to the latest or a specific version
  opencode uninstall           uninstall opencode and remove all related files
  opencode serve               starts a headless opencode server
  opencode web                 start opencode server and open web interface
  opencode models [provider]   list all available models
  opencode stats               show token usage and cost statistics
  opencode export [sessionID]  export session data as JSON
  opencode import <file>       import session data from JSON file or URL
  opencode github              manage GitHub agent
  opencode pr <number>         fetch and checkout a GitHub PR branch, then run opencode
  opencode session             manage sessions
  opencode db                  database tools

Positionals:
  project  path to start opencode in                                                        [string]

Options:
  -h, --help         show help                                                             [boolean]
  -v, --version      show version number                                                   [boolean]
      --print-logs   print logs to stderr                                                  [boolean]
      --log-level    log level                  [string] [choices: "DEBUG", "INFO", "WARN", "ERROR"]
      --port         port to listen on                                         [number] [default: 0]
      --hostname     hostname to listen on                           [string] [default: "127.0.0.1"]
      --mdns         enable mDNS service discovery (defaults hostname to 0.0.0.0)
                                                                          [boolean] [default: false]
      --mdns-domain  custom domain name for mDNS service (default: opencode.local)
                                                                [string] [default: "opencode.local"]
      --cors         additional domains to allow for CORS                      [array] [default: []]
  -m, --model        model to use in the format of provider/model                           [string]
  -c, --continue     continue the last session                                             [boolean]
  -s, --session      session id to continue                                                 [string]
      --fork         fork the session when continuing (use with --continue or --session)   [boolean]
      --prompt       prompt to use                                                          [string]
      --agent        agent to use                                                           [string]