PIPE-3211: update pixi tasks guide and environment specs
Update pixi developer documentation to align with recent task and environment configurations:
- Document INIT_CWD working directory semantics and thread pool clamping for casa and casampi tasks.
- Add missing bundle-assets and update-references task descriptions.
- Remove duplicate environment table entry and update dated references.
- Sync conda migration examples with openmpi-mpicxx and casatasks pins.
- Clean up the common tasks quick reference table.
