feat(init): ask to link org during hm init (CLI-39)

[markovejnovic]

Summary

• Adds a cloud organization linking flow to hm init — after template selection (TTY-only), users can register their repo with Harmont Cloud
• If not logged in, offers to trigger the browser-loopback login flow first
• On org selection, writes a sparse .hm/config.toml with backend = "cloud" and the org slug
• hm run automatically prefers cloud when the project config is present (existing figment layering, no hm run code changes needed)
• Exposes login_interactive() from hm-plugin-cloud for embedding in host commands

UX Flow

Logged in:

Link this repo to Harmont Cloud?:
> No, skip
  Foo Org (foo-org)
  Harmont-Dev (harmont-dev)

Not logged in:

You are not logged in to Harmont Cloud. Log in now? [y/N]

→ If yes, opens browser login, then shows org selector above.

Test plan

• Non-interactive init (--template rust, no TTY) skips cloud registration — no config.toml created
• Sparse cloud project config round-trips through figment correctly
• Existing config layering test confirms backend = "cloud" in project file wins over user-level backend = "docker"
• All 24 existing init tests still pass
• Clippy clean

Closes CLI-39
Closes #108