feat: add seamless templates list --json for template discovery

[Bccorb]

Problem

There is no way to discover what seamless init can scaffold without reading source. The
registry lives in the seamless-templates monorepo and is fetched at scaffold time
(src/core/templates.ts); nothing surfaces its contents. To learn that
the React Vite starter is react-vite behind the flag --basic, or that the api layer has
express and fastify with no aliases at all, you have to open registry.json.

That makes the CLI hard to drive from a script or an agent, and it makes the alias mismatch
(next issue) invisible.

Change

Add a templates command:

seamless templates list [--json]

• Table output: id, kind, framework, label, flag (the --<alias> form, blank when the
  template has no alias), status.
• --json emits the registry entries as an array, for scripting parity with
  apps list --json.
• Honors SEAMLESS_TEMPLATES_DIR and SEAMLESS_TEMPLATES_REF like init does, so it
  reports the same set that a scaffold would use.
• coming-soon entries are listed and marked, matching how the prompts render them.
• Add the command to COMMAND_HELP in src/commands/helpTopics.ts
  so it picks up --help and the dispatcher's known-command list.

Acceptance

• seamless templates list prints every registry entry without a portal session.
• seamless templates list --json output parses and contains id, kind, alias, and status.
• Registry fetch failure produces the same actionable error init gives, not a stack trace.
• Referenced from the init help in place of the "run an unknown flag" hint.