## The bug
`GET /api/command-center/tokens` documents and accepts `groupBy=task`,
but the `task` dimension was never actually wired through the
aggregator. Four enumeration sites all stopped at `model | provider |
node | agent`:
- `TokenGroupBy` (core `token-analytics.ts`) did not include `"task"`.
- `groupKeyFor()` had no `case "task"`.
- `VALID_GROUP_BY` (dashboard route) rejected the value, so
`resolveGroupBy()` returned `undefined`.
- `groupAttributes()` (core `otel-metrics.ts`) emitted no attribute for
it.
Net effect: a caller asking for a per-task rollup silently fell back to
**ungrouped grand totals** — the per-task breakdown returned zero
groups, even though every task carries `tokenUsage*` and the data was
right there.
## The fix
Thread `"task"` through all four sites (5 functional lines + doc/test):
- Add `"task"` to the `TokenGroupBy` union — this makes the two `switch`
statements **compiler-exhaustive**, so `tsc` forces the two new cases
(no silent gaps).
- `groupKeyFor`: task rows group by their task id; chat rows have no
task and return `null`, mirroring the existing `node` case.
- `groupAttributes`: emit `task.id` for OTLP export, matching `node.id`
/ `agent.id`.
- `VALID_GROUP_BY`: accept `"task"`.
No schema or migration change — the task id is already on the row.
## Verification
- `pnpm --filter @fusion/core typecheck` and `@fusion/dashboard
typecheck` — clean.
- Extended the existing `groups by provider, node, agent` core test with
a `groupBy: "task"` assertion (two tasks → two groups keyed by task id,
100 / 200 tokens). Full suites green: **core 25/25**, **dashboard
325/325**.
<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit
* **New Features**
* Added task-based token analytics grouping, alongside existing grouping
options.
* Analytics views and metrics now include task-level breakdowns when
available.
* **Bug Fixes**
* Improved grouping behavior so task totals are reported correctly in
analytics results.
* **Documentation**
* Updated supported analytics options to reflect task grouping in
endpoint and metric descriptions.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->
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