fix(opencode): add OpenCode 2 compatibility

[NidAll]

Summary

Update the OpenCode harness for OpenCode 2 compatibility.

OpenCode 2 removed the --pure flag and changed the custom agent configuration schema, causing current Ask releases to fail with:

Unrecognized flag: --pure in command opencode

This updates the harness to use the OpenCode 2 CLI and configuration format while preserving Ask's read-only behavior.

Changes

• replace opencode --pure run with the OpenCode 2 run --standalone flow
• replace opencode --pure models with opencode models
• migrate custom agent configuration from agent to agents
• migrate agent instructions from prompt to system
• migrate the old permission object to OpenCode 2 ordered permissions
• preserve the read-only default-deny policy
• keep .env files protected while allowing .env.example
• update OpenCode harness tests for the V2 configuration and CLI
• preserve session continuation and JSON event parsing

Testing

Tested locally with OpenCode 2 using Ask end-to-end.

running 85 tests
test result: ok. 85 passed; 0 failed

Also verified:

• one-shot questions work through OpenCode 2
• model selection works through ask --settings
• OpenCode model discovery works
• session continuation works
• read-only agent configuration is injected correctly

The upstream OpenCode 1-style harness failed before this change because OpenCode 2 no longer recognizes --pure.