Add Project Validation - Pre Execution

When executing commands, the CLI may require some files to be available, top examples:

1. `main.pipeline.jsonnet`.
2. `cleanup.pipeline.jsonnet`.

These files should be validated pre-execution and have a clear error message requiring them.
E.g.:
```
no such file: `main.pipeline.jsonnet`
`shore render` can only be invoked if `main.pipeline.jsonnet` exists.
Please validate `main.pipeline.jsonnet` exist in the root of the project.

no such file: `cleanup.pipeline.jsonnet`
`shore render` can only be invoked if `cleanup.pipeline.jsonnet` exists.
Please validate `cleanup.pipeline.jsonnet` exist in the root of the project.
```

Start at the CLI command execution path for `shore render` and trace where `main.pipeline.jsonnet` and `cleanup.pipeline.jsonnet` are required. Add pre-execution validation with clear missing-file messages, then verify the command behavior when each file is absent.

Written by the indexing model from the issue text.