apps init: allow scaffolding into the current directory

[calvarjorge]

What changes

Non-interactive: --name .

• databricks apps init --name . scaffolds directly into the current
  working directory.
• The app name is derived from the cwd basename and validated against the
  existing project-name rules ([a-z][a-z0-9-]*, ≤ 26 chars). An invalid
  basename produces an actionable error pointing at renaming the directory.
• The cwd must be empty other than .git/ and .gitignore, so the common
  mkdir foo && cd foo && git init flow still works while avoiding any
  risk of overwriting existing user files.
• --name . and --output-dir are mutually exclusive (one already names
  the destination).
• The cd <name> line in the success / next-steps output is omitted when
  scaffolding in place.

Interactive

When the cwd is eligible for in-place use (empty modulo .git/.gitignore,
valid basename), the user is asked up front:

Where should we create the app?
▸ Create a new subdirectory (default)
Use the current directory ("foo")

The default focused option is "Create a new subdirectory", so anyone who
hits Enter through prompts gets today's behaviour. The in-place option is
opt-in but discoverable. If the cwd isn't eligible, the existing name
prompt runs unchanged.

Test plan

Build the CLI from this branch:

```
go build -o ./cli
```

Then try the new flow:

```

In-place scaffold (non-interactive)

mkdir /tmp/my-app && cd /tmp/my-app
git init
~/path/to/cli apps init --name .
ls -A # files land at the top level, no nested my-app/

In-place scaffold (interactive)

mkdir /tmp/foo && cd /tmp/foo
~/path/to/cli apps init # first prompt is the new selector
```

Unit tests: `go test ./cmd/apps/... ./libs/apps/prompt/...`.