Use last Cloud workspace and sidebar order for new workspace targeting

[austinywang]

Cmd+Shift+Y now creates a new Cloud workspace on the machine that owns the most recently selected Cloud workspace in the originating window. Switching to a local workspace preserves that Cloud context. If the remembered workspace is missing, deleted, or outside the active account/team, the action uses the first machine in the Cloud sidebar's existing pin/reorder presentation order.

Implements issue #13184.

Behavior and ownership changes:

• Removed the default/primary Cloud machine store, star, menu actions, snapshot fields, and routing. Legacy cloud.defaultMachineID is retired on startup without changing machine pins, order, data, or lifecycle.
• Cmd+Y remains the deliberate New Cloud Machine action. Cmd+N keeps its explicit current-machine behavior. Existing shortcut IDs, settings customization, menu labels, and Cloud access gates remain intact.
• The target resolver is shared by shortcut/menu/configuration paths and reads the window-owned last Cloud selection plus the account/team-scoped CloudMachinePinStore order. It never independently sorts an API response or revives a legacy default.
• Cloud creation captures window, scope, and selection revision. It admits the local Cloud pane and input relay before remote creation, binds the machine immediately, then attaches the remote receipt. Completion cannot focus over newer navigation; cancellation and late receipts clean only operation-owned remote resources.
• Empty fleets produce a localized explicit message and leave New Cloud Machine available separately.

Validation:

• Hosted GitHub CI is green, including macOS compile admission, workflow guards, lifecycle, web validation, and test routing; PR head is 181790cd63a4271a4860d55020869df29e69af9d.
• Canonical $autoreview is clean with no actionable or Aziz policy findings.
• Local non-build checks pass: Swift file budgets (no TSV edits), localization catalog structure/parity, pbxproj normalization, test wiring, web JSON parsing, and git diff --check.
• Focused behavior coverage covers last-selection targeting, local switching, machine switching, pin/reorder fallback, stale/deleted/cross-team selection, empty fleets, legacy preference disagreement, shortcut routing, window ownership, early pane admission, retry ownership, and suspended-receipt cancellation cleanup.
• Controller fleet job 01f1b2644887c30de15e3168 built the exact pushed SHA with tag issue-13184-cloud-workspace-targeting. The terminal receipt and submitted receipt are preserved under artifacts/fleet/; the artifact was verified as cmux DEV issue-13184-cloud-workspace-targeting.app with the matching isolated bundle ID and SHA-256 b41e8f157a662e34e78483f09d9ed7fc79c535c455c8fc927c73d5743756866c.

The requested merge from the latest origin/main (ad34966d0641) was completed and pushed. No Depot, local Apple build, production VM, billing, or deployment change was used.