CLI Extensions — Stage 1: Extensible edgezero-cli library + generator + app-demo-cli skeleton #260

Part of the EdgeZero CLI Extensions effort — stage 1 of 8.

Scope: Turn edgezero-cli into lib + bin: add src/lib.rs with the public run_* API and standalone #[derive(clap::Args)] structs; shrink main.rs to a thin wrapper. Rename the dev subcommand to demo (dev is reserved for a future dev-workflow command); dev_server.rs → demo_server.rs, run_dev → run_demo() -> Result<(), String>. Extend the generator to scaffold a <name>-cli crate (new templates/cli/). Add a handwritten examples/app-demo/crates/app-demo-cli parallel.

No dependency on PR #253.

Ship gate: existing edgezero commands keep the same flags; app-demo-cli --help shows the five built-ins; edgezero new <tmp> && cargo check --workspace succeeds; all four CI gates green.

Design spec: docs/superpowers/specs/2026-05-19-cli-extensions-design.md §7
Implementation plan: docs/superpowers/plans/2026-05-20-cli-extensions.md (Stage 1)