docs: document rebuild_queue dispatch for approval kinds (#124, #441 followup)

docs/approvals.md

@@ -187,6 +187,49 @@ approval id to retry. Because tags are first-class git objects,
 rejected and failed trees stay browsable forever — `git log
 --tags` in the applied repo is the audit trail.
 
+### Dispatch via `rebuild_queue` (#441)
+
+Long-running approval work — `ApplyCommit`, `UpdateMetaInputs`,
+`Spawn` — no longer runs inline inside `actions::approve`. Instead
+the approval handler enqueues a `QueueEntry` into the global
+`rebuild_queue`:
+
+| `ApprovalKind` | `QueueKind` queued | `QueueSource` |
+|---|---|---|
+| `ApplyCommit` | `Rebuild` | `Approval` |
+| `UpdateMetaInputs` | `MetaUpdate` | `Approval` |
+| `Spawn` | `Spawn` | `Approval` |
+| `InitConfig` | — runs inline (sub-second git seed) | — |
+| `SchedulePrompt` | — runs inline (single sqlite insert) | — |
+
+Each queue entry carries the originating `approval_id` so the
+worker can re-fetch the approval row when it dispatches, run the
+kind-specific pipeline (`run_approval_apply_commit` /
+`run_approval_update_meta_inputs` / `run_approval_spawn`), and
+fire the matching `HelperEvent::*` on completion via
+`finish_approval`.
+
+Two visible consequences:
+
+- **Operator dashboard**: after clicking APPR0VE the work-in-progress
+  shows up on the *rebuild queue* card (`POST /api/state.rebuild_queue`
+  + live `rebuild_queue_changed` events), not on the approvals panel
+  (which already moved the row to "approved"). A long meta-update
+  cascade renders as a parent entry with one child per per-agent
+  rebuild — see `docs/web-ui.md` for the layout.
+- **Cancellation**: the dashboard's *× cancel* button on a `Queued`
+  entry calls `POST /api/rebuild-queue/{id}/cancel`, which flips the
+  entry to `Cancelled` before the worker dispatches it. Returns
+  `{"cancelled": true}` on success, `{"cancelled": false}` if the
+  entry already left `Queued` (running / done / failed) — terminal
+  states can't be retroactively rewritten.
+
+`QueueSource::Approval` carries the `approval_id` so a tail-end
+build failure surfaces back as a failed approval row, not just a
+silent queue entry. `QueueSource::Manual` (dashboard ↻ R3BU1LD)
+and `QueueSource::AutoUpdate` (boot-time sweep) use the same
+queue but skip the approval row plumbing.
+
 ### Forge mirror
 
 When the bundled `hive-forge` container is running — on by