Refactor runtime launches into adapter registry

[ericlewis]

Summary

• introduce a runtime adapter registry and shared launch contract (request + plan + capabilities)
• move runtime-specific launch logic into dedicated adapters (claude, codex-app-server, amp)
• keep dispatcher.py as a compatibility wrapper that delegates to adapters
• wire Engine to launch via an injected/selected runtime adapter
• extend CLI runtime selection with aliases (--amp, --claude, --codex) and runtime-aware default models
• update docs for runtime options and harness structure

Why

This keeps engine/runtime concerns separated, makes adding backends cheaper, and makes runtime capabilities explicit.

Notes

• amp runtime currently warns when --model/--budget are provided because Amp does not support those flags in this integration.
• did not run tests in this change set.