refactor(model): move AWS-specific fields to metadata structs

[mpyw]

Summary

• Move provider-specific fields from base types to metadata structs for multi-cloud support
• Parameter.Type → AWSParameterMeta.Type
• ParameterListItem.Type → AWSParameterListItemMeta.Type (new)
• Secret.ARN → AWSSecretMeta.ARN
• SecretListItem.ARN, DeletedDate → AWSSecretListItemMeta (new)
• Add AWSMeta() helper methods for type-safe metadata access

Test plan

• make build passes
• make test passes
• make lint passes

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 14.63415% with 35 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.87%. Comparing base (53ecc39) to head (61500ff).
⚠️ Report is 1 commits behind head on multicloud.

Files with missing lines	Patch %	Lines
internal/model/parameter.go	25.00%	9 Missing ⚠️
internal/model/secret.go	25.00%	9 Missing ⚠️
internal/provider/aws/secret/adapter.go	0.00%	9 Missing ⚠️
internal/provider/aws/param/adapter.go	0.00%	8 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff               @@
##           multicloud     #151      +/-   ##
==============================================
- Coverage       87.14%   86.87%   -0.28%     
==============================================
  Files             139      139              
  Lines            8170     8198      +28     
==============================================
+ Hits             7120     7122       +2     
- Misses            806      831      +25     
- Partials          244      245       +1     

Flag	Coverage Δ
e2e	64.53% <0.00%> (-0.23%)	⬇️
platform-darwin	64.38% <ø> (ø)
platform-linux	62.85% <ø> (ø)
platform-windows	73.46% <ø> (ø)
unittests	76.19% <14.63%> (-0.24%)	⬇️

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