feat: support merging multiple sources

[rmarganti]

Summary

• allow repeated --from args and preset from arrays
• merge secrets from multiple sources before sync or diff
• reject conflicting duplicate keys while allowing identical duplicates
• add config schema 1.1.0 for multi-source presets while preserving 1.0.0
• update default config schema reference and README documentation

Validation

• cargo fmt
• cargo test
• cargo clippy -- -D warnings