refactor: retire env-var credential path for proxmox and opnsense providers

[endavis]

Description

Retire the environment-variable credential path for Proxmox and OPNsense providers. Both providers now receive credentials exclusively through SOPS-decrypted .tfvars files, eliminating the redundant CredentialLoader -> os.environ -> TF_VAR_* pipeline that was a legacy holdover.

Addresses #554

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Code refactoring
• Performance improvement
• Test improvement

Changes Made

• Deleted proxmox_loader.py and opnsense_loader.py credential loaders
• Removed _CREDENTIAL_ENV_MAPPING and os.environ lookup from ProxmoxProvider.get_terraform_env_vars() and OPNsenseProvider.get_terraform_env_vars()
• Removed Proxmox/OPNsense entries from CredentialLoader.PROVIDER_LOADERS registry
• Cleaned up credential_loader/__init__.py exports
• Simplified Proxmox variables.tf.j2 descriptions (removed stale env-var references)
• Updated documentation: credential-loader-system.md, ci-cd-deployment.md, ci-cd-with-separate-config-repo.md, ENV_EXAMPLE.md, ENVRC_LOCAL.md, direnv.md
• Updated tests to use Kubernetes provider examples instead of removed Proxmox/OPNsense loaders

Testing

• All existing tests pass (doit check passes)
• Updated test_credential_loader.py and test_cli_credentials.py to remove references to deleted loaders
• Manually verified no remaining references to the deleted modules

Checklist

• My code follows the code style of this project (ran doit format)
• I have run linting checks (doit lint)
• I have run type checking (doit type_check)
• I have added tests that prove my fix is effective or that my feature works
• All new and existing tests pass (doit test)
• I have updated the documentation accordingly
• I have updated the CHANGELOG.md
• My changes generate no new warnings

Additional Notes

This is a breaking change for anyone relying on PROXMOX_API_URL, PROXMOX_API_TOKEN_ID, PROXMOX_API_TOKEN_SECRET, OPNSENSE_API_URL, OPNSENSE_API_KEY, or OPNSENSE_API_SECRET environment variables to pass credentials. The .tfvars-based path (via SOPS secrets) is now the only supported mechanism.

[github-actions]

⚠️ API Breaking Changes Detected

The following breaking changes were detected by griffe:

src/infrafoundry/core/credential_loader/credential_loader.py:41: CredentialLoader.PROVIDER_LOADERS:
Attribute value was changed:
  Old: {'proxmox': ProxmoxCredentialLoader, 'opnsense': OPNsenseCredentialLoader, 'kubernetes': KubernetesCredentialLoader}
  New: {'kubernetes': KubernetesCredentialLoader}

src/infrafoundry/core/credential_loader/__init__.py:0: OPNsenseCredentialLoader:
Public object was removed

src/infrafoundry/core/credential_loader/__init__.py:0: ProxmoxCredentialLoader:
Public object was removed

src/infrafoundry/core/credential_loader/proxmox_loader.py:0: <module>:
Public object was removed

src/infrafoundry/core/credential_loader/opnsense_loader.py:0: <module>:
Public object was removed

✅ Breaking Change Documented

This PR includes breaking change documentation.

Before merging, verify:

• Migration guide in CHANGELOG.md
• Documentation updated
• Version will be bumped appropriately (major version)

[github-actions]

Benchmark

Details

Benchmark suite	Current: b461d86	Previous: 75f3d27	Ratio
tests/benchmarks/test_placeholder.py::test_import_time	7454.4991992532505 iter/sec (stddev: 0.000009200307612621936)	9439.317642212467 iter/sec (stddev: 0.000016589072149731227)	1.27

This comment was automatically generated by workflow using github-action-benchmark.

[github-actions]

⚠️ Performance Alert ⚠️

Possible performance regression was detected for benchmark.
Benchmark result of this commit is worse than the previous benchmark result exceeding threshold 1.10.

Benchmark suite	Current: b461d86	Previous: 75f3d27	Ratio
tests/benchmarks/test_placeholder.py::test_import_time	7454.4991992532505 iter/sec (stddev: 0.000009200307612621936)	9439.317642212467 iter/sec (stddev: 0.000016589072149731227)	1.27

This comment was automatically generated by workflow using github-action-benchmark.