Move AlgebraicMultigrid to a package extension

[Copilot]

This change makes AlgebraicMultigrid optional at base package load time by moving its integration behind a Julia package extension. AMG functionality remains available when AlgebraicMultigrid is present, without forcing it as a hard dependency of Jutul.

• Dependency model update

  • Moved AlgebraicMultigrid from [deps] to [weakdeps] in Project.toml.
  • Added extension registration:
    • JutulAlgebraicMultigridExt = "AlgebraicMultigrid"
  • Added AlgebraicMultigrid to test extras/targets so AMG tests still execute.

• Core package decoupling

  • Removed unconditional using AlgebraicMultigrid from src/Jutul.jl.
  • Removed unconditional AMG implementation include from src/linsolve/precond/precond.jl.
  • Added src/ext/algebraicmultigrid_ext.jl with core-side AMG preconditioner interface and extension availability hooks.

• Extension-backed AMG implementation

  • Added ext/JutulAlgebraicMultigridExt/JutulAlgebraicMultigridExt.jl.
  • Implemented extension hooks (check_algebraicmultigrid_availability_impl, default AMG cycle).
  • Loaded the existing AMG implementation from within the extension so it is only activated when AlgebraicMultigrid is loaded.

• Test path alignment

  • Updated AMG test module imports so extension loading is explicit in the AlgebraicMultigrid test path.

[weakdeps]
AlgebraicMultigrid = "2169fc97-5a83-5252-b627-83903c6c433c"

[extensions]
JutulAlgebraicMultigridExt = "AlgebraicMultigrid"