feat(#2485): remove vestigial Noop + StartupSweep residuals

Since the boot sweep (#2450) and meta-update cascade (#2476) became
single DAGs that grow subgraphs in-place, nothing constructs the old
fan-out anchors/parents anymore:

- NodeKind::Noop (the old boot_root grouping anchor) — no constructors.
- Template::StartupSweep / Source::StartupSweep (the old fan-out parent
  template + cascade-child source) — replaced by Template::Boot and
  Source::AutoUpdate/MetaUpdate respectively.

Drops the three variants + their as_str arms + the Noop executor arm, and
refreshes the stale fan-out/anchor doc comments (Boot/MetaUpdate/Source
docs, coordinator.md, dashboard.md). Frontend: the queue-kind glyph moves
from the dead startup_sweep to boot (which had none), and the dead
rqe-source-startup_sweep style is dropped.

No behaviour change — pure dead-variant removal.

docs/coordinator.md

@@ -241,10 +241,10 @@ On startup, `auto_update::run` classifies every agent by rev freshness (the
 per-agent `.{name}.hyperhive-rev` marker under `/var/lib/hyperhive/applied/`
 vs the current flake path) and persisted `wanted` intent, then:
 
-1. **Config path** — when *any* marker is stale, submit one `StartupSweep`
-   DAG: a `MetaLock` (hyperhive input bump, non-fatal) that fans out `Rebuild`
-   children for the stale agents whose `wanted = Up` (topology-sorted, parents
-   first). Stale but wanted-offline agents get no boot-time nix work — their
+1. **Config path** — when *any* marker is stale, submit one `Boot`
+   DAG: a `MetaLock` (hyperhive input bump, non-fatal) that grows an in-DAG
+   `Rebuild` subgraph for each stale agent whose `wanted = Up` (topology-sorted,
+   parents first). Stale but wanted-offline agents get no boot-time nix work — their
    rebuild happens on their next start (the start submit path upgrades a
    stale start to rebuild+start), which is also why the lock bump runs even
    when every stale agent is offline: those later start-upgrades must build