feat(cli): the scaffold ships one starter blueprint

Task 6 of the bound-override cycle: SIGNAL_OPEN_JSON_STD and its write
retire — aura new scaffolds signal.json alone, and the project CLAUDE.md
quickstart advertises run AND sweep against that one file (the sweep line
overrides the bound fast.length; --list-axes shows open + bound axes).
The data-only e2e loop proves it end to end: scaffold, run the closed
starter, sweep it via an override axis, and assert signal.json is the
only starter blueprint. The authoring-guide's starter-pair sentence is
retargeted; the last signal_open reference outside the unrelated #196
fixture string is gone.

refs #246

docs/authoring-guide.md

@@ -38,9 +38,10 @@ the same shape every node in `aura-std` already follows.
 - **The project's own signal doesn't need a crate by default.** `aura new`
   scaffolds a **data-only** project (`docs/project-layout.md`, "A project
   repo (two tiers)") — a strategy over the std vocabulary is a
-  blueprint/campaign document, not Rust: the scaffold ships a closed `signal.json` run target
-  and an open `signal_open.json` sweep target, both runnable immediately, no
-  build step.
+  blueprint/campaign document, not Rust: the scaffold ships a single closed
+  `signal.json` starter, runnable immediately with no build step, and also
+  the sweep target itself (a bound param is a default any `--axis` may
+  override, #246).
 - **A project-specific *native* node type** — the moment §0's three-part
   pattern is actually needed — lives in an attached **node crate**:
   `aura nodes new <name>` scaffolds a sibling cdylib crate (`Cargo.toml` +