Spec: Keep window open after closing the last tab (APP-5230)

[warp-agent-staging]

Description

Defines the product and technical contract for making final-tab window closing configurable. The revised spec makes the replacement active-tab invariant explicit as Option<usize>, audits 267 non-test invariant locations across 159 functions, specifies safe behavior by call-site class, raises the true zero-tab variant to L (5), and contrasts it with APP-5242's lower-risk fresh-tab approach. It also covers settings and command-palette surfaces, confirmation ownership, CloseWindow-disabled hosts, persistence, recovery paths, and the intentionally asymmetric single-tab drag behavior.

This draft PR will carry the implementation after the spec is approved.

Originating thread: https://warpdev.slack.com/archives/C0BDQDW8V5E/p1786043046497489

Linked Issue

APP-5230

Testing

• ./script/format --check
• Presubmit clippy configurations for the workspace, Warp app, and warp_completer
• Manual GUI testing — deferred to implementation; the spec requires a runnable Dogfood build and computer-use video proof

Screenshots / Videos

Not applicable to this spec-only commit. Implementation must attach the required visual proof before this PR is marked ready.

Agent Mode

• Warp Agent Mode - This PR was created via Warp's AI Agent Mode

Co-Authored-By: Warp Agent agent@warp.dev

Conversation: https://staging.warp.dev/conversation/dfbf2c3d-65c2-432b-ba26-9e3384f53e81
Run: https://oz.staging.warp.dev/runs/019fdc72-0204-7987-ab2f-1b286324f7e5

This PR was generated with Oz.