Inconsistent working directory: docs generation and "add entity" require different CWDs #7

What happened

Summary

Two core commands expect the user to stand in two different directories:

• Generating MDL documentation only works inside the model folder (my-model/).
• Adding a new entity only works one level above the model folder (outside my-model/).

There is no single directory from which I can do both, so I have to keep
cd-ing back and forth between the project root and the model folder.

Environment / layout

modelith/                 <- project root (one level above the model)
└── my-model/             <- the model
    ├── mdl-project.yaml
    ├── .mdl/
    ├── conceptual/
    │   ├── entities/
    │   └── subject-areas/
    └── logical/
        ├── domains/
        └── entities/

Steps to reproduce

1. cd modelith/my-model

2. Generate documentation -> works.

3. Try to add a new entity from the same directory -> fails with:

   no mdl-project.yml in my-model
   

4. cd .. (project root, outside my-model/)

5. Add a new entity -> works.

6. Try to generate documentation from the same directory -> fails with:

   Error: /api/model: 404 {"detail":"no mdl-project.yaml in ."}
   

What you expected

Expected behaviour

Every command should work from inside the model folder (my-model/), which is
where the model actually lives and where mdl-project.yaml sits. Ideally the
project root is discovered by walking up from the current directory until an
mdl-project.yaml is found, so both commands work from anywhere inside the
model tree.

Actual behaviour

• Docs generation resolves the project relative to the current directory
  (.) and 404s when run from the parent folder.
• Add-entity resolves the project relative to the parent directory and fails
  when run from inside the model folder.

Notes / suspected cause

• Looks like a relative-path bug: the two code paths use different base
  directories when resolving the project file, and neither of them walks up
  the directory tree.
• The two error messages also disagree on the file name: one says
  mdl-project.yml, the other mdl-project.yaml. The file on disk is
  mdl-project.yaml. If the add-entity path really looks for .yml, the
  error message is misleading at best and a second bug at worst.
• The no mdl-project.yml in my-model message is confusing on its own,
  because my-model/mdl-project.yaml clearly does exist.

Desired outcome

I want to work exclusively inside my own model folder: adding entities,
generating documentation and everything else should run from my-model/
without having to change directories.

Modelith version

0.1.1

Environment

macOS Tahoe 26.3, Python 3.12.11

Anything else

No response