chore: publish the corpus as @taskless/cli/reference.json

[thecodedrift]

Renames the published subpath from @taskless/cli/demo-reference.json to
@taskless/cli/reference.json, and follows that through the code.

Why

The name carried "demo" into something consumers type, and into modules whose
subject is broader than the demo command. The sg and vale entries are
reference material too — they are simply not what taskless demo writes. So
scoping the corpus to "demo" was wrong in the same way the published name was.

Moves, by what each file is about

src/rules/demo/reference.ts   -> src/rules/reference.ts
src/rules/demo/constraints.ts -> src/rules/constraints.ts
assets/demo-reference.json    -> assets/reference.json

The constraints move furthest and had the least business being under demo/:
they describe what verify and test enforce on any rule. Nothing about
them is demonstration-specific, and the old location implied a scope they never
had.

src/rules/demo/ keeps manifest.ts and rule.ts — those really are the rules
the demo command writes.

Identifiers and tasks follow: buildDemoReference → buildReference,
DemoReference → Reference, DEMO_REFERENCE_* → REFERENCE_*,
write-demo-reference.ts → write-reference.ts, demo:reference →
reference. Tests renamed to match the modules they cover.

demo:manifests and print-demo-manifests.ts keep the prefix, and that is
the line: they exist so CI can stage what the demo command writes.

The changeset is amended, not added to

It has not been released — the Version Packages PR is still open, and the only
artifact carrying the old name is last night's nightly, republished from main
on every merge. So the release note describes the name that will actually ship,
and one change still gets one note.

Verified

• The payload is byte-identical. Regenerated after the move; it matched.
• The asset guard still fires under the new path — removed files and
  watched the build fail:

  "./reference.json" points at ./assets/reference.json, which no "files"
  entry ships — it would resolve here and nowhere else
  

• The tarball carries it: npm notice 12.8kB assets/reference.json
• The export resolves: ./reference.json -> ./assets/reference.json,
  rules: sg,vale,runtime, constraints: 7
• 1267 tests, lint and typecheck clean.

No behaviour change.