Add experimental multi-machine picker

[ymichael]

Human comments

What was wrong

The environment picker grouped every available environment under every machine, so adding machines repeated options and pushed frequently used choices below the fold. The machine-only picker also had no way to narrow a long host list.

What changed

• Added a searchable, target-first environment picker for projects with at least three machines. Physical machines and hostless machine providers share a capped Machines section, while the selected physical machine's environments appear in a separate section.
• Made machine selection update the composer's actual host immediately without closing the picker. The tab's current environment option is reused across machines, falls back to the first usable option, and remains explicit when the target needs project setup.
• Preserved offline and project-setup states, aligned target icons, and gave the selected machine and its environment sub-selection independent checked states.
• Added search to machine-only pickers with more than five machines.
• Put both behaviors behind the default-off multiMachinePicker experiment, exposed through Settings and bb settings experiment multiMachinePicker <true|false>.
• Updated stories, tests, system-config fixtures, CLI coverage, and the configuration/BB guide docs. No server/daemon wire contract changed.

Environment picker

Before:

After:

Machine picker

Before:

After searching:

How you verified

• pnpm exec turbo run test --filter=@bb/app -- --run src/components/pickers/EnvironmentPicker.test.tsx src/components/plugin/PluginNewThreadComposer.test.tsx src/views/RootComposeView.test.ts (144 tests)
• Focused experiment contract tests for @bb/server, @bb/domain, @bb/db, @bb/cli, and @bb/desktop (16 tests)
• pnpm exec turbo run typecheck --filter=@bb/app --filter=@bb/server --filter=@bb/domain --filter=@bb/db --filter=@bb/cli
• Live browser verification with the experiment disabled and enabled, using connected, offline, project-not-configured, and hostless machine fixtures. The rebased build also verified immediate host selection, an open menu after selection, and same-tab selection restoration after reload.

Fixes #3562

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