feat: accept template ids as init flags (--react-vite alongside --basic)

[Bccorb]

Problem

The template flag and the template id are different strings, and only the flag works.

From registry.json:

id	alias	flag that works today
react-vite	basic	--basic
react-oauth	oauth	--oauth
express	(none)	none
fastify	(none)	none

So someone asked for the React Vite starter types --react-vite, gets
Unknown option "--react-vite", and has no way to find --basic short of reading the
registry. The api layer is worse: neither express nor fastify has an alias, so there is no
flag for the backend at all and the prompt cannot be skipped.

resolveTemplateAliases() in src/commands/init.ts matches on
t.alias only.

Change

• Match a flag against the template id as well as its alias, so --react-vite and
  --basic both select react-vite, and --express / --fastify start working with no
  registry change.
• Keep aliases as the short form; they stay in the help text and the docs.
• Extend the "Available template flags" error to list both forms, and point at
  seamless templates list.
• Conflict detection is unchanged: two flags resolving to different templates of the same
  kind is still a hard error, and --basic --react-vite resolves to the same id so it is not
  a conflict.

Acceptance

• seamless init --react-vite and seamless init --basic produce identical preselection.
• seamless init --fastify preselects the api template and skips the backend prompt.
• seamless init --basic --react-oauth still errors as a conflict.
• Unknown flag error names both the id and alias forms.