fix: name the unreachable host when a scaffold fetch fails

[Bccorb]

Problem

seamless init reported a bare Error: fetch failed whenever one of its network reads was
rejected at the connection level, giving no way to tell what was unreachable:

Creating project in /tmp/.../smoke
Error: fetch failed

There are three remote reads on the scaffold path, and the message was identical for all three:

1. openRemoteSource in src/core/templates.ts, fetching registry.json
2. ensureArchive in the same file, downloading the templates .zip
3. fetchEnvExample in src/core/fetch.ts, fetching the auth server's .env.example

Each already handles a non-ok HTTP response with a message naming the status and the URL. What
was missing is the connection-level rejection: fetch throws TypeError: fetch failed and that
propagated untouched to the top-level handler in src/index.ts.

This showed up repeatedly while building the scaffold smoke job (#192).

Change

A fetchRemote(url, purpose) helper in src/core/fetch.ts wraps the GET and turns a
connection-level rejection into a message naming the URL, what the CLI wanted from it, and the
network as the likely cause, in the style loginFlow.ts's request() already uses. The original
error is kept as the thrown error's cause. All three call sites go through it. Non-ok responses
keep the messages they had.

Verification

npm run build and npm test pass (979 tests). New unit tests cover all three messages, and each
path was also exercised end to end against the built CLI with only the failing host blocked:

Error: Could not reach https://raw.githubusercontent.com/fells-code/seamless-templates/v0.9.0/registry.json to read the template registry. Check your network connection.
Error: Could not reach https://github.com/fells-code/seamless-templates/archive/v0.9.0.zip to download the project templates. Check your network connection.
Error: Could not reach https://raw.githubusercontent.com/fells-code/seamless-auth-api/v0.7.1/.env.example to read the auth server's env.example. Check your network connection.

Changeset: patch.