refactor(effect): use SessionRevert service in prompt

[kitlangton]

Summary

• replace the remaining SessionPrompt -> SessionRevert.cleanup(...) facade hop with direct SessionRevert.Service usage
• provide SessionRevert.defaultLayer where SessionPrompt is composed, including manual session test layer stacks
• replace lazy Layer.unwrap(Effect.sync(() => ...)) wrappers with Layer.suspend(() => ...) in the prompt/session stack and tool registry

Notes

• SessionPrompt.cancel(...) in tool/task.ts still stays on the facade path because a direct SessionPrompt.Service dependency there would create a SessionPrompt -> ToolRegistry -> TaskTool -> SessionPrompt cycle

Testing

• bun typecheck
• bun turbo typecheck
• bun run test -- test/session/prompt-effect.test.ts test/session/snapshot-tool-race.test.ts