feat: unified source identity (RFC 0009)

[rmanibus]

Summary

Implement RFC 0009 by introducing a unified source identity model across supported source types.

What Changes

• Add/adjust source identity fields and formatting in core/engine paths.
• Update backup/list/policy flows to use consistent source identity semantics.
• Update source implementations to provide stable identity details:
  • local
  • Google Drive
  • OneDrive
  • SFTP
• Update docs and RFC index:
  • rfcs/0009-unified-source-identity.md
  • docs/sources.md
  • docs/user-guide.md

Behavior Notes

• Source grouping and display should be more consistent across commands.
• Identity fields are populated with stable account/container/path context where available.

Testing

• Includes updates to source tests and volume identity related tests.

Tracking

• Story / epic: RFC 0009: Epic / Tracking issue for unified source identity #98
• RFC: rfcs/0009-unified-source-identity.md
• Milestone: RFC 0009: Unified source identity

[codecov]

Codecov Report

❌ Patch coverage is 53.70370% with 75 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pkg/source/onedrive.go	17.77%	34 Missing and 3 partials ⚠️
pkg/source/gdrive.go	60.00%	8 Missing and 2 partials ⚠️
internal/engine/policy.go	59.09%	7 Missing and 2 partials ⚠️
internal/engine/list.go	0.00%	8 Missing ⚠️
internal/engine/backup.go	65.00%	6 Missing and 1 partial ⚠️
cmd/cloudstic/format.go	76.47%	2 Missing and 2 partials ⚠️

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