Fix path dialogs, shell-search inference, and toast dismissal

[SawyerHood]

Human comments

What was wrong

Three small, independent problems from mbullington's agent/garden-upstream-stack, rebased onto main:

• Project path dialogs (new project, change workspace, add machine) assumed the local platform. On a remote host with a different OS the separator, root, and home heuristics were wrong.
• The thread view inferred a "search" intent from any shell command that mentioned grep/rg, so an unrelated bash call rendered as a search row.
• Raw Sonner toasts (plugin toasts, provider notifications) had no close button, and AppToastContent never rendered one for custom content.

What changed

• apps/app path dialogs and useLocalPathPicker take the selected host's platform; HostResponse exposes it (packages/server-contract/src/api/hosts.ts, packages/sdk/src/areas/hosts.ts). Offline hosts fall back to the prop platform.
• packages/thread-view/src/tool-call-parsing.ts stops inferring search intents from shell text; tests and CLI rendering snapshots updated. The handoff notes live in plans/pi-session-tool-precedence-and-search-intents.md.
• apps/app/src/main.tsx enables Sonner's closeButton; AppToastContent renders its own dismiss X.

No wire change. Bottom of the stack: no prerequisite.

How you verified

pnpm exec turbo run typecheck on this layer, plus turbo run test for @bb/app, @bb/server, @bb/server-contract, @bb/sdk, @bb/thread-view, @bb/mobile, @bb/host-daemon (see the stack validation log in the thread).

Fixes: none (upstream stack split).

AGENT GENERATED