feat(staging): split state.json into param.json and secret.json

[mpyw]

Summary

• Refactor file-based staging store to use separate files per service
• param.json for Parameter Store entries
• secret.json for Secrets Manager entries

Changes

• Store now uses stateDir instead of stateFilePath
• Add NewStoreWithDir constructor, deprecate NewStoreWithPath
• Drain merges state from both service files
• WriteState writes to appropriate file(s) based on service filter
• Delete removes both files
• Exists returns true if either file exists
• IsEncrypted checks both files

Why

This separation prepares for multi-cloud support where different providers may have different service availability (e.g., GCP only has Secret Manager, no Parameter Store equivalent).

Test plan

• Unit tests updated and passing
• Lint passing
• E2E tests (manual verification)

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 98.83721% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 90.14%. Comparing base (1253dff) to head (3b1b4f2).
⚠️ Report is 1 commits behind head on multicloud.

Files with missing lines	Patch %	Lines
internal/staging/store/file/store.go	98.83%	1 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff               @@
##           multicloud     #144      +/-   ##
==============================================
+ Coverage       90.08%   90.14%   +0.06%     
==============================================
  Files             133      133              
  Lines            7754     7807      +53     
==============================================
+ Hits             6985     7038      +53     
  Misses            526      526              
  Partials          243      243              

Flag	Coverage Δ
e2e	67.48% <52.32%> (-0.02%)	⬇️
platform-darwin	64.38% <ø> (ø)
platform-linux	62.85% <ø> (ø)
platform-windows	73.46% <ø> (ø)
unittests	79.67% <98.83%> (+0.13%)	⬆️

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