task board: add a To-do column, restrict the + add-task button to To-do only, and auto-assign+link an agent on move-to-in_progress

[oxoxDev]

Ask (from product)

Three changes to the task board:

1. Add a "To-do" column. Keep Backlog; add To-do as a distinct column. (Current columns: `backlog, in_progress, paused, in_review, done` — `frontend/src/lib/tasks-sample.ts:21-28`, backend constants `src/harness/lifecycle.rs:47-56`.) Confirm the intended order/semantics of Backlog vs To-do with product (this issue assumes both exist; new tasks land in To-do).

2. Restrict the "+" add-task button to the To-do column only. Today it renders on every column unconditionally — `frontend/src/views/TasksView.tsx:233-239` (inside the `TASK_COLUMNS.map`). Only To-do should show it.

3. On move-to-in_progress, the orchestrator assigns AND links an agent. When a card enters in_progress with no explicit (or invalid) assignee, the orchestrator should pick a valid agent, write it to `card.assignee` (link it), and dispatch — the working agent must be visible on the card. Today the dispatch falls back to the orchestrator silently and never writes `assignee` (see tasks: the working agent is never linked to the card's assignee; blank/invalid/overlay assignees silently fall back to the orchestrator with no validation #205). The in_progress edge fires at `src/company/runtime.rs:32-39` → `dispatch_task` @361-385 → `run_task`.

Notes

• Adding a column touches: `TASK_COLUMNS` (frontend), any backend column constants + the create default (`src/server/ops/tasks.rs:211` defaults to `backlog` → likely `todo`), GraphQL/docs (`src/server/graphql/tasks.rs:21`, `schema.graphql:651`).
• Part 3 overlaps the fix in tasks: the working agent is never linked to the card's assignee; blank/invalid/overlay assignees silently fall back to the orchestrator with no validation #205 (assignee linking) — coordinate.

Found while UI-testing the consolidated build (e2e_harness co).