Document vault migrate command in reference docs

The vault migrate command was added in PR #1150 but the reference documentation at content/manual/reference/vaults.md does not yet cover it.

What needs updating

1. CLI Commands section (vaults.md)

Add a new ### swamp vault migrate <vault_name> section after swamp vault type search documenting:

• Arguments: <vault_name> (required)
• Options:
  • --to-type <type> (required) — Target vault type
  • --config <json> — Provider-specific config as JSON (e.g. '{"region":"us-east-1"}')
  • --dry-run — Preview migration without making changes
  • -f, --force — Skip confirmation prompt
  • --repo-dir <dir> — Repository directory (default .)
• Example output for both log and JSON modes
• Dry-run example showing the preview output

2. Vault Types section (vaults.md)

Add a note that vaults can be migrated between types in-place using swamp vault migrate, preserving the vault name so all existing vault.get() references continue working.

3. Migration behavior notes

• Secrets are copied to the target backend first, then the config is swapped (save-new-before-delete-old for safety)
• Same-type migrations are rejected
• The vault name and ID are preserved — only the type and config change
• All existing CEL expressions (vault.get('vault-name', 'key')) continue working without modification

Context

This was implemented in swamp PR #1150 (systeminit/swamp#1150). The command is available in the latest swamp build.