feat(xtask): scaffold generic hemx apps

Add cargo run -p hemx-xtask -- app new PATH for the generic checked-hypermedia starter: v0 page/form/keyed-row/notice handlers and tests with generated helpers instead of raw ids, opcodes, selector UI JavaScript, or manual registry plumbing.

req: ceremony/005

req: canonical_authoring/003

AGENTS.md

@@ -54,6 +54,7 @@ Keep it stable. Prefer pointers to canonical sources over copied structure, file
 - hemx core stays small: effects, typed ids, registries, and wire schema only.
 - Routing, auth, sessions, transport, transitions, sync, and storage belong in integration/user crates.
 - Public examples and beginner APIs should use generated resources and `IntoEffect`, not raw ids or runtime opcodes.
+- Use `cargo run -p hemx-xtask -- app new PATH` for the generic page/form/keyed-row/notice starter; use `workout new PATH` only for the phone-first Workout product starter. req: ceremony/005
 - The public component-reuse explanation lives in `docs/recipes/reusable-partials.md`; do not grow a client component framework to explain partial composition.
 - Hemlate examples must use real hemplate syntax, not Vue/Handlebars sketches: `{+ expr +}` for escaped text, `{+= expr =+}` only for trusted/rendered HTML, `+attr="expr"` for dynamic attributes, and Rust-shaped `h-if`, `h-for`, `h-match`, `h-case` directives (`h-case="_"` is the default arm).
 - JS runtime changes must preserve root-scoped lookup and avoid selectors, VDOM, expressions, and per-node listeners.