feat: task decomposition view during execution (#5)

At the start of job execution, a Haiku call decomposes the user's
request into 2-4 named sub-steps. Steps are broadcast via WebSocket
as `job_steps` and `job_step_update` events. The Workshop renders a
live checklist panel near Gamma that checks off steps as streaming
progresses using a character-count heuristic, then collapses to
"Done" on completion. Steps are stored with the job record.

- agent.ts: add `decomposeRequest` (Haiku, stub-safe)
- event-bus.ts: add `DecompositionEvent` types (job:steps, job:step_update)
- jobs.ts: call decompose before streaming; advance steps heuristically
- events.ts: translate new bus events to WS messages
- websocket.js: handle job_steps / job_step_update / collapse on complete
- ui.js: showJobSteps / updateJobStep / collapseJobSteps panel
- jobs schema: decomposition_steps text column
- migration 0010: add decomposition_steps column

Fixes #5

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/db/migrations/0010_task_decomposition.sql

@@ -0,0 +1,4 @@
+-- Task decomposition view (#5)
+-- Stores the Haiku-generated step labels for a job execution as a JSON array.
+
+ALTER TABLE jobs ADD COLUMN IF NOT EXISTS decomposition_steps TEXT;