Add Heroku-style `git:remote` command to create a git remote for an app

[usiegl00]

Summary

Add a Heroku-style git:remote command (and optionally auto-add a remote on apps:create) so users can wire up a git remote for deploying via git push, without manually copying the git URL.

This is a follow-up to #29, which added the git URL to apps:create output. The convention in this repo's skills docs is a remote named bld (see src/commands/skills.cr:58: git remote add bld "$GIT_URL").

Motivation

Today, after creating an app, a user has to manually run:

GIT_URL=$(bld apps:info -a my-app -j | jq -r '.git_url')
git remote add bld "$GIT_URL"
git push bld main

Heroku collapses this into heroku git:remote -a my-app (and heroku create auto-adds a heroku remote). We should match that ergonomics.

Proposed behavior

New command: bld git:remote

• Usage: bld git:remote [APP] [-a APP] [--remote NAME]
• Resolves the app from the positional APP argument or the -a/--app option (same pattern as apps:info in src/commands/app.cr).
• Fetches the app via api.app(app_input) and reads app.git_url.
• Must be run inside a git work tree (git rev-parse --is-inside-work-tree); otherwise fail with a clear message.
• Default remote name is bld (override with --remote NAME).
• If the remote already exists, update it (git remote set-url); otherwise add it (git remote add). Report which action was taken and the URL.
• Fail clearly if the app has no git_url.

Example:

$ bld git:remote -a my-app
Added git remote 'bld'. Deploy with: git push bld main

Optional: auto-add remote on apps:create

To mirror heroku create, when apps:create runs inside a git repository:

• Auto-add a bld remote pointing at the new app's git_url.
• Add --remote NAME to choose the name and --no-remote to opt out.
• Print a one-line confirmation (e.g. Added git remote 'bld'. Deploy with: git push bld main).

Decide during implementation whether auto-add-on-create is desired or whether the explicit git:remote command alone is preferable (auto-add changes existing apps:create behavior).

Implementation notes

• Add shared git helpers to Build::Commands::Base (src/commands/base.cr), e.g. git_repo?, git_remote_exists?(name), set_git_remote(name, url), each shelling out via Process.run("git", [...], output: Close, error: Close). Use the args-array form (not shell: true) to avoid quoting/injection issues.
• Register the new command in src/build_cli.cr alongside the other App::* commands.
• The App model already exposes git_url (build-client 1.1.37), so no SDK changes are needed.

Acceptance criteria

• bld git:remote -a my-app adds a bld remote pointing at the app's git URL inside a repo.
• --remote NAME overrides the remote name.
• Running an existing remote name updates it rather than erroring.
• Clear error when not in a git repo or when the app has no git URL.
• Decision recorded on whether apps:create auto-adds a remote (with --no-remote/--remote if so).