refactor(cost): single writer for cost_entries, finishRun reads SUM

Cost was written through three independent paths: worker-runner shipped
`finalUsage.costUsd` in run-finished -> finishRun wrote `runs.cost_usd`,
`tasks.cost_usd += costUsd`, `agents.spent_monthly_usd += costUsd`, and
inserted a per-run aggregate row in cost_entries. CostWriter
additionally inserted per-message rows in cost_entries from main. Any
CostWriter failure created silent drift between `SUM(cost_entries)`
(read by budget enforcement) and `runs.cost_usd` (read by the UI).

Make CostWriter the single writer, finishRun the single reader.

- Add `db.recordCostEntry(...)` as the first-class insert path for
  per-message cost rows. CostWriter calls it; tests call it directly.
- Remove the aggregate INSERT from finishRun. finishRun now reads
  `SELECT COALESCE(SUM(amount_usd), 0) FROM cost_entries WHERE run_id=?
  AND unattributed=0` and writes that as the authoritative run total to
  runs.cost_usd, tasks.cost_usd, and agents.spent_monthly_usd.
- Drop `costUsd` from `finishRun` input, the worker `run-finished`
  ChildMessage shape, the automation `RunFinishedEvent` type, and every
  caller + test fixture. Cost is owned by the per-message path.
- worker-runner stops extracting `usage.cost.total` — token counts stay
  for session accumulation, cost comes from pi-ai via CostWriter.
- run-finished-helpers now gates budget incident checks on
  `event.status === "succeeded"` instead of a cost field that no longer
  exists on the event.
- automation-run-completion fires `budget_threshold` on every succeeded
  run (post-run spent_monthly_usd already reflects the SUM path).
- Update budget tests to write a `recordCostEntry` before `finishRun`
  to simulate the per-message cost path; drop `costUsd` from cross-
  subsystem, edge-case-budget, database, and related fixtures.

One writer, one reader, one source of truth. No drift.

src/main/orchestration/task-runner.ts

@@ -182,7 +182,6 @@ export function createTaskRunner(
               signal: null,
               model: null,
               sessionDisplayId: null,
-              costUsd: null,
               unattributedCost: true,
             });
             db.addActivity({