Keep archived worktrees optimistically hidden

[boudra]

Summary

• Keep the app's optimistic worktree archive hide/rollback path intact so the initiating client does not show the row again while the archive RPC is pending.
• Add server-owned archivingAt metadata to workspace descriptors so archive-in-progress upserts are explicit and backward-compatible.
• Suppress local archivingAt workspace upserts only when this client has the matching archive action pending; other clients can still render the archiving spinner, and failure restores the hidden row.

Test plan

• npm run format:check
• npm run typecheck
• npm run lint
• npm run test --workspace=@getpaseo/app -- src/stores/checkout-git-actions-store.test.ts src/contexts/session-workspace-upserts.test.ts src/components/sidebar-workspace-list.test.tsx src/stores/session-store.test.ts --bail=1
• cd packages/server && npx vitest run src/server/agent/mcp-server.test.ts src/server/session.workspaces.test.ts src/server/worktree-session.test.ts src/shared/messages.workspaces.test.ts --bail=1