Build the walking skeleton: upload a file to a container

## What

Build the walking skeleton of the uploader: one command that takes a file path, uploads it to a container, and reports what happened.

This is the first slice of #1. It deliberately does not answer what happens when the blob already exists, because that decision is still open on the parent.

## Why this slice first

It proves the whole path end to end, from argument parsing through configuration binding to a real write against the emulator. Every later slice is a policy change on top of a path that already works.

## How

- Create the solution, the library project, the CLI project, and the test project against `net8.0`
- Define `IBlobUploader` with `ExistsAsync` and `UploadAsync`, and implement it over `BlobContainerClient`
- Add `UploadService` with an `UploadAsync` that opens the file, writes it, and returns an `UploadResult`
- Bind `UploaderOptions` from `appsettings.json` with `UseDevelopmentStorage=true`
- Parse `` and optional `--container` in `Program.cs`
- Return exit code 0 on success and 2 on failure, leaving 1 reserved
- Create the container if it does not exist, so a first run on a clean emulator works
- Keep the connection string out of every message and log line

Assume the container is empty. Existing blob handling is out of scope for this slice.

## Out of scope

Existing blob handling, retry and backoff, chunked upload, SAS generation, server side encryption, container lifecycle policies, real Azure authentication.

## Testing

- [ ] Policy test against the `IBlobUploader` fake: an upload into an empty container succeeds and reports the byte count
- [ ] Policy test: a missing source file fails with an actionable message, not a stack trace
- [ ] Policy test: no captured output contains the connection string
- [ ] The opt-in Azurite integration test uploads and then reads the blob back
- [ ] `dotnet test --filter "Category!=Integration"` passes with no emulator running

## Done when

Someone who has just cloned the repository can start Azurite, run one command, and see the file land in the container.

## Estimate

S, 3 hours (set at 09:50).

Start with the solution structure, Program.cs argument handling, and appsettings.json configuration, then trace UploadService through IBlobUploader and BlobContainerClient. Add the policy tests and opt-in Azurite integration test described in the issue, and verify with dotnet test --filter "Category!=Integration" plus a real upload and readback.

Written by the indexing model from the issue text.