Add 'backlog' task status for non-dispatchable task tracking

[thomaswillner]

Problem

Currently, all tasks created in Mission Control are automatically routed and dispatched to agents through the autoRouteInboxTasks() → dispatchAssignedTasks() pipeline. There is no way to create a task that simply sits as a tracking item / reminder / backlog entry without being picked up by the scheduler.

The current status flow is:

inbox → assigned → in_progress → review → quality_review → done

Any task entering inbox with assigned_to IS NULL gets auto-routed to the best available agent, moved to assigned, and dispatched. Setting priority to low only affects ordering, not filtering — low-priority tasks still get dispatched.

Proposed Solution

Add a backlog status that:

• Is visible as a column on the Task Board (before Inbox)
• Is excluded from autoRouteInboxTasks() and dispatchAssignedTasks()
• Can be manually moved to inbox when ready for dispatch (drag-and-drop on board)
• Maps to backlog in GNAP sync
• Has a GitHub label mc:backlog

Files that need changes:

1. src/lib/validation.ts — add backlog to Zod status enums
2. src/lib/github-label-map.ts — add to TaskStatus type and STATUS_LABEL_MAP
3. src/lib/gnap-sync.ts — add bidirectional mapping
4. src/components/panels/task-board-panel.tsx — add column + Task type + dropdown option
5. messages/*.json — add colBacklog i18n key
6. No changes needed to task-dispatch.ts — dispatch already only queries inbox and assigned

Use Case

As a user, I want to track future work items (e.g., "Switch voice provider from Twilio to Telnyx") without the system immediately trying to execute them. These are reminders / deferred tasks that I'll manually promote to inbox when ready.

Workaround

Currently using assigned_to: "_hold" with status inbox to prevent auto-routing (since the auto-router requires assigned_to IS NULL), but this is fragile and semantically wrong.

[0xNyk]

Implemented in #544. The backlog status is now available:

• Added to Zod validation schemas (createTaskSchema, bulkUpdateTaskStatusSchema)
• New Kanban column "Backlog" appears before Inbox on the task board
• GitHub label mc:backlog (color: slate) added to STATUS_LABEL_MAP
• GNAP sync maps backlog ↔ backlog bidirectionally
• i18n keys added for all 10 locales
• Task dispatch is unaffected — autoRouteInboxTasks() only queries inbox status, so backlog tasks are automatically excluded
• TUI updated with backlog as valid status