Add a create store option to the app dev store picker

[dmerand]

Important

Stacked PR. Base is dlm-app-dev-inline-store-creation (#8323), not main. This PR only shows the store-picker option on top of the inline creation flow that #8323 adds. Do not merge before #8323.

WHY are these changes introduced?

Closes shop/issues-develop#23597

#8323 lets app dev create a dev store inline, but only when the organization has zero stores. Developers with existing stores still have to leave the CLI to create a new one. Per the command-scope decision, the store picker in app dev should also offer a "create store" option.

WHAT is this pull request doing?

Adds a Create a new dev store choice to the app dev store picker for App Management organizations that already have stores. Selecting it runs the same name/plan/create/poll flow that #8323 added for the zero-store case.

• New StoreCreationMode value 'selection-option'; only app dev opts in. All other callers keep the default 'disabled', and 'when-empty' behavior is unchanged.
• With one or more stores, the picker shows every store plus the create choice. One store no longer auto-selects when the create choice is available. The choice persists through picker search.
• Zero stores still create directly (no picker), as in Create dev stores from app dev #8323.
• Cap handling: with existing stores, a capped organization keeps normal store selection and cancellation; the create choice is hidden. Zero-store cap guidance is unchanged. The cap query still fails open and never runs for Partners or disabled callers.
• Non-interactive runs never prompt or create. Zero stores, multiple returned stores, or a returned page with more stores available fails with --store guidance. One confirmed store still auto-selects, and explicit or cached selection still bypasses the picker.
• Partners keeps the dashboard link/reload flow.

How to test your changes?

1. Link an app in an App Management organization that has at least one dev store.
2. Run app dev --reset. The store picker lists your stores plus Create a new dev store.
3. Pick an existing store: dev starts as before.
4. Run app dev --reset again and pick Create a new dev store: enter a name, pick a plan, and dev continues on the new store after the success banner.

Manual tophat: this picker-entry behavior was tophatted before the stack split, on the pre-split equivalent SHA 4e85e16c (multi-store list with create choice, name/basic plan, single success banner, preview ready, explicit --store and cached-store bypass, store list stays dev-type, standalone store create dev --json unaffected, cleanup). The current child SHA has not been live-tophatted yet.

Checklist

• I've considered possible cross-platform impacts (Mac, Linux, Windows)
• I've considered possible documentation changes
• I've considered analytics changes to measure impact
• The change is user-facing — I've identified the correct bump type (patch for bug fixes · minor for new features · major for breaking changes) and added a changeset with pnpm changeset add