Pin work_products identity to (company, workspace, path)

The natural key used to be (companyId, provider, externalId), so two
workspaces with the same workspace-relative path collided onto one
row. Provenance was also frozen at the first write: `created_by_run_id`
never refreshed, so a run that edited a pre-existing file never
qualified as a deliverable and the DocumentCenter "open" link could
still jump into the first workspace after a repo move.

Migration v45 adds `workspace_id` + `last_touched_by_run_id`, backfills
`workspace_id` via the task/run join, seeds `last_touched_by_run_id =
created_by_run_id`, and installs a partial unique index on
`(company_id, workspace_id, provider, external_id)` scoped to rows
that have a workspace. The artifact projector now records
`workspace_id` on insert and refreshes `last_touched_by_run_id` on
every upsert. The deliverable signal query reads either provenance
column so a run that only edited a pre-existing file still counts.
`resolveWorkProductTarget` prefers the stored `workspace_id` and falls
back to the task/run join for legacy NULL rows.

src/main/worker/worker-runner.ts

@@ -120,6 +120,7 @@ export class WorkerRunner {
         companyId: job.companyId,
         taskId: job.taskId,
         modelId: job.model,
+        workspaceId: job.workspaceId ?? null,
         workspacePath: job.workspacePath ?? null,
       });
       if (!prepareResponse.ok) {