feat: add multi-agent collaboration tabs

[H1d3rOne]

Summary

• Open delegated sub-agent conversations in workspace tabs next to the current tab, with labels derived from their responsibility/task.
• Add a tile/single-tab view toggle so multiple delegated agent tabs can be viewed side by side or one at a time.
• Persist delegated tab title overrides in opened_tabs so tabs recover their role names after restart instead of staying at Loading.

Verification

• pnpm eslint src/contexts/tab-context.tsx src/contexts/tab-context.test.tsx src/lib/types.ts src/hooks/use-open-conversation-tab.ts
• pnpm test src/contexts/tab-context.test.tsx
• cd src-tauri && cargo test --features test-utils save_opened_tabs_core_persists_only_conversation_tabs_and_bumps_version
• git diff --check

Notes

• Local dev port config changes are intentionally not included in this PR.