feat: add CLI passthrough flags for Claude session options

[frankekn]

Reference #92

This is a maintainer-owned replacement PR for the current-architecture version of the Claude session-option passthrough work.

Why replace the original branch:

• the original fork PR is currently blocked on GitHub Actions action_required
• the code itself rebases cleanly to current main
• local gates are green on the rebased head

Included behavior:

• add first-class CLI flags for --model, --allowed-tools, and --max-turns
• thread those flags through exec, sessions new, and sessions ensure
• forward Claude-specific session options via ACP session/new _meta.claudeCode.options
• include parsing/help/unit/integration coverage for the passthrough path

Local validation:

• pnpm run format:check
• pnpm run typecheck
• pnpm run build
• pnpm run build:test && node --test dist-test/test/client.test.js dist-test/test/cli.test.js dist-test/test/integration.test.js