feat(plugin): add v2 effect host

[thdxr]

Summary

• add the public @opencode-ai/plugin/v2/effect host and SDK-backed domain contracts
• make core domain transforms effectful, replayable, scoped, and disposable
• adapt plugin boot and built-in plugins to register through the V2 host
• expand tests for transform replay, disposal, plugin hosts, and migrated domains

Testing

• bun turbo typecheck (pre-push hook)

[greptile-apps]

Too many files changed for review. (104 files found, 100 file limit)