Gates the CLI first-run prompt (from #53663) on the `agent-v2` feature
flag, giving us an escape hatch if something goes wrong.

## What this does

**Commit 1: Gate on agent-v2 flag**
- Adds an `AgentV2FeatureFlag` check at the top of the open behavior
resolution. When the flag is off, the prompt is skipped.
- Refactors `maybe_prompt_open_behavior` → `resolve_open_behavior`:
moves the explicit-flags guard (`-n`, `-e`, `--reuse`) to the caller and
drops three parameters from the function signature, since the function
was only inspecting those values to decide whether to bail early.

**Commit 2: Default to new-window when flag is disabled**
- When the flag is off, falls back to the pre-#53254 behavior of opening
projects in a new window, rather than silently adding to the sidebar.

Release Notes:

- N/A