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/web-ui/dashboard.md

@@ -207,9 +207,9 @@ One operation runs at a time; the worker drains FIFO. Each row
 shows a state glyph (`⏸` queued / `▶` running / `✔` done /
 `✖` failed / `⊘` cancelled), kind glyph + verb (`↻ rebuild`,
 `◆ meta_update`, `✨ spawn`, `🗑 destroy`, `↺ restart`,
-`⚡ startup_sweep`, `🔑 perm_change`, `⏹ graceful_stop`, `▶ start`,
+`⚡ boot`, `🔑 perm_change`, `⏹ graceful_stop`, `▶ start`,
 `■ stop`), agent name,
-source chip (`manual | meta_update | auto_update | startup_sweep | crash_recover | approval`
+source chip (`manual | meta_update | auto_update | crash_recover | approval`
 — green for operator-approved config changes), timing, and an
 optional reason / error. A multi-step op is a single DAG: its
 per-agent subgraphs and sub-steps (a meta-update cascade's per-agent