[lavaman131]

The Add project dialog could only add: choosing a repository added it and
closed the dialog, and one already added showed greyed out with nothing to
do. Nothing anywhere could take a repository out of the Projects sidebar.

The dialog now works like GitHub's multi-select repository picker (Primer's
SelectPanel): each row is a checkbox and the repository's full name,
projects listed first as it opens, and choosing a row (click, Enter or
Space) adds or removes it at once. The list never reorders after a choice.
It stays open for several changes and closes with Escape or a click
outside. With no projects yet, the first one added still closes it, so a
first run goes straight to creating a document. A row being changed shows
a spinner in its checkbox; a failure restores it and says why.

• Server: NavigationStore.removeProject (memory and PostgreSQL, one storage
  contract), and DELETE /api/navigation/projects/:repositoryId (same-origin,
  signed in, 204 or 404). Removing the repository of the last document
  forgets that document. Opening a document there adds the project back.
• Web: PaletteListbox takes checked, set as aria-checked, while
  aria-selected keeps meaning the highlighted row. The checkbox is drawn as
  choice-control's, inside the option, so no control nests in another.
  The sidebar moves focus to a new project only when the dialog closes
  with it; while the dialog stays open it keeps focus. removeProject lives
  in the lazily loaded dialog, keeping the initial JavaScript within budget.
• e2e/machine-choice.e2e.ts closes the dialog with Escape after its second
  and third projects, as it now stays open.

Assistant-model: Claude Opus 5.5 (fast)
Assistant-workflow: inline
Assistant-verification: Chromium E2E passed: machine-choice, hosted, document-creation, auth-lifecycle, keyboard-shortcuts and shell suites (102 pass), and machine-choice's "creating projects" 8 of 8 with --repeat-each 4, which failed in CI before focus stayed in the open dialog
Assistant-verification: bun test unit passed: apps/web/src (847 pass), src/navigation (10 pass), storage contract in memory and against postgres:17
Assistant-verification: typecheck, lint and build passed: bun run types, bun run ci, bun run build (initial JS 257905 B, under the 258000 B budget)
User-preference: Model the project picker on GitHub's repository selection UI with checkboxes, and remove anything excessive
Co-authored-by: Alex Lavaee lavaman131@github.com