build: add .tool-versions for mise / asdf

Pins the development toolchain so contributors get the same versions
the maintainer uses:

  python 3.12
  uv 0.11.8
  just 1.50.0

Compatible with both mise (https://mise.jdx.dev) and asdf
(https://asdf-vm.com). Install one of those, run 'mise install' or
'asdf install' in the repo root, and you're on the same Python /
uv / just versions as CI.

Pure addition: no impact on contributors who don't use mise/asdf.
The CI workflow installs Python 3.10/3.11/3.12/3.13 in matrix; this
file just locks the local-dev default to 3.12 so 'python --version'
in the repo gives a predictable answer.

Author: Pawansingh3889

.tool-versions

@@ -0,0 +1,3 @@
+python 3.12
+uv 0.11.8
+just 1.50.0