Add ability to define mounts during start

[myles-anderson-ox]

Add --mount support to nebubox start

Why: nebubox start only ever mounted the project directory and the tool's auth files, with no way to expose additional host paths to the container. Anything else a session needed, such as other repositories in other directories, was inaccessible. This is particularly helpful on MacOS where mount --bind does not exist and there is not another well supported, secure way to do it.

What changed:

• Added a repeatable --mount [:][:ro|rw] flag to nebubox start.
• The destination is optional and defaults to /home/coder/workspace/ (alongside the project). A relative dest is placed under the workspace; an absolute dest (e.g. /home/coder/.aws) is used verbatim. The middle segment is treated as the mode only when it's exactly ro/rw.
• Host paths are validated (must exist; resolved to absolute) via a new validateMount helper, and parseArgs now accumulates repeated flags so --mount can be given multiple times.
• Mounts are applied at container creation; starting an existing container warns that --rebuild is needed to change them.
• Updated help text, README, and CHANGELOG; added unit tests for parsing, validation (all shorthand forms), and the container -v wiring.