feat: vaults v2 enhanced secret management

[jahvon]

Complete overhaul of the vault system introducing named vaults, multiple encryption backends, and improved workflows for managing secrets across different vaults.

New Features

Named Vault System

• Multiple Vaults: Create and manage multiple named vaults instead of a single global vault
• Vault Switching: Switch between vaults using flow vault switch <name>
• Vault Management: Full lifecycle management with create, list, get, remove, and switch commands
• Cross-Vault Access: Retrieve secrets from specific vaults without switching using vault/secret-name syntax

Multiple Encryption Backends

• AES256 (Default): Symmetric encryption with auto-generated 256-bit keys
• Age Support: Asymmetric encryption using https://github.com/FiloSottile/age with recipient keys for team collaboration
• Key Sharing: Share encryption keys across multiple AES256 vaults for related vaults

Enhanced Authentication

• Flexible Key Storage: Configure keys via environment variables or files during vault creation
• Custom Environment Variables: Specify custom env vars for vault keys (--key-env, --identity-env)
• Identity Files: Support for Age identity files and AES256 key files

Advanced Configuration

• Custom Storage Paths: Specify custom storage locations for vault data (--path flag)
• Provider Architecture: Extensible provider system for different vault backends
• Migration Support: Migrate pre-v1 legacy vaults to the new named system

Secret Management

• Context-Aware: Secrets are managed within the currently active vault
• Isolated Storage: Each vault maintains its own encrypted secret store
• Backward Compatibility: Existing secret commands work with the legacy vault system

[codecov]

Codecov Report

Attention: Patch coverage is 3.06886% with 1295 lines in your changes missing coverage. Please review.

Project coverage is 18.82%. Comparing base (ed36313) to head (f12851b).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
cmd/internal/vault.go	0.00%	345 Missing ⚠️
internal/vault/v2/secret.go	0.00%	210 Missing ⚠️
internal/vault/v2/vault.go	0.00%	139 Missing ⚠️
internal/io/secret/v2/views.go	0.00%	138 Missing ⚠️
cmd/internal/secret.go	0.00%	119 Missing ⚠️
internal/io/vault/view.go	0.00%	109 Missing ⚠️
internal/vault/v2/demo.go	0.00%	89 Missing ⚠️
internal/io/vault/output.go	0.00%	48 Missing ⚠️
internal/runner/env.go	12.12%	29 Missing ⚠️
internal/io/secret/v2/output.go	0.00%	25 Missing ⚠️
... and 8 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #246      +/-   ##
==========================================
- Coverage   21.86%   18.82%   -3.05%     
==========================================
  Files          84       91       +7     
  Lines        7106     8339    +1233     
==========================================
+ Hits         1554     1570      +16     
- Misses       5269     6478    +1209     
- Partials      283      291       +8     

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