feat(kanban): enable direct task delegation to external agents (#109)

[Lexus2016]

Summary

Addresses Phase 1 of #109 by enabling task delegation directly from the Kanban board without requiring a pre-existing conversation:

1. Kanban Direct Delegation:
   • Added a delegate button (⇗) to task card hover actions and the Task Edit modal footer.
   • Built a dedicated delegation dialog in public/kanban.html allowing users to select an external agent (Codex, Claude, etc. from /api/external-agents), choose execution mode (handoff or sync), pick model/effort if configured, and fine-tune task instructions prefilled from the card title and description.
2. Backend Task Linking:
   • Updated /api/delegate in server.js to accept taskId and custom workdir.
   • Automatically associates task context/session, marks backlog/todo tasks as in_progress, and records taskId in active delegation state.
   • Reported taskId in GET /api/delegate/status.
3. Live Visibility & Dialog Viewer:
   • Delegated cards display an active badge (⇗ <AgentName>) with a live spin indicator.
   • Clicking the badge opens the delegation status dialog displaying the active agent, duration, live output (DIALOG.md), follow-up message input, and stop control.
4. i18n & Automated Regression Tests:
   • Added dlg.* dictionary entries across all 5 supported languages (uk, en, ru, fr, he), verified by test/i18n-completeness.test.js.
   • Added automated tests in test/kanban-delegation.test.js.

Related to #109