Refactor execution targets into plugins and add HPC targets

[vornit]

Summary

This PR refactors q8s execution targets to be discovered through Python entry points and moves target-specific implementations into separate plugin packages.

Changes:

• Add entry point based execution target discovery through the q8s.targets entry point group
• Keep cpu as the built-in execution target in the core q8s package
• Move CUDA support into the separate q8s-cuda plugin package
• Update tests and move CUDA-specific tests to the q8s-cuda plugin package
• Add new HPC plugin packages:
  • q8s-hpc-cpu, providing the hpc-cpu execution target
  • q8s-hpc-rocm, providing the hpc-rocm execution target
• Update package metadata for the core package and plugin packages
• Add README and LICENSE files for the publishable packages
• Add an ADR for entry point based execution targets
• Bump the core q8s package version to 0.14.0

Testing

• Built all packages successfully
• Verified entry point based target discovery
• Ran existing tests

[codecov]

Codecov Report

❌ Patch coverage is 63.20755% with 39 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
packages/q8s/src/q8s/cli.py	0.00%	18 Missing ⚠️
packages/q8s/src/q8s/project.py	66.66%	10 Missing ⚠️
packages/q8s/src/q8s/execution.py	70.00%	6 Missing ⚠️
packages/q8s/src/q8s/kernel.py	40.00%	3 Missing ⚠️
packages/q8s/src/q8s/targets.py	83.33%	2 Missing ⚠️

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