fix: honor --model for reconnect prompts

[skywills]

Summary

This PR fixes model selection drift on persistent/reconnected sessions so per-prompt --model is honored consistently (including warm queue-owner paths).

Root Cause

Session options were not consistently propagated through refactored queue-owner request/task boundaries, and model config updates were not always enforced immediately before the next prompt on all adapters.

Changes

• Ported session-option propagation to refactored runtime/queue files.
• Added merge helpers so command-level session options override persisted defaults where appropriate.
• Carried per-prompt sessionOptions through queue submit request parsing, transport, and task execution.
• Ensured queued prompt execution prefers task-level session options over owner startup defaults.
• Improved model switching behavior:
  • prefer session/set_config_option for model updates (alias-friendly)
  • immediately enforce with session/set_model after config update when available
  • gracefully tolerate adapters that do not support specific session control methods
• Added/updated integration coverage for prompt model updates on existing sessions.

Files Changed

• src/acp/client.ts
• src/acp/session-control-errors.ts
• src/cli/command-handlers.ts
• src/cli/queue/ipc-server.ts
• src/cli/queue/ipc.ts
• src/cli/queue/messages.ts
• src/cli/queue/owner-env.ts
• src/cli/session/contracts.ts
• src/cli/session/queue-owner-process.ts
• src/cli/session/queue-owner-runtime.ts
• src/cli/session/runtime.ts
• src/runtime/engine/connected-session.ts
• src/runtime/engine/session-options.ts
• test/integration.test.ts

Validation

• corepack pnpm run typecheck
• corepack pnpm run build:test
• node --test --test-name-pattern "integration: prompt --model updates existing session model before prompt" dist-test/test/integration.test.js

Manual Repro

• npx tsx src/cli.ts --model haiku claude sessions ensure --name "acpx-test-model"
• npx tsx src/cli.ts --model haiku claude prompt -s acpx-test-model "What is your model name"
• npx tsx src/cli.ts --verbose --model sonnet claude prompt -s acpx-test-model "What is your model name"
• Confirmed model switches correctly on subsequent prompt turns.