Adds support for alternative AI runtime backends (Codex, OpenCode) alongside
the default Claude backend through a runtime abstraction layer.
- internal/runtime/runtime.go - Runtime-agnostic helper functions
- Extended RuntimeConfig with provider-specific settings
- internal/opencode/ for OpenCode plugin support
- Updated session managers to use runtime abstraction
- Removed unused ensureXxxSession functions
- Fixed daemon.go indentation, updated terminology to runtime
Backward compatible: Claude remains default runtime.
Co-Authored-By: Ben Kraus <ben@cinematicsoftware.com>
Co-Authored-By: Cameron Palmer <cameronmpalmer@users.noreply.github.com>
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