git commit -m feat: added evn to CLI flags

[Jolah1]

Closes #5

Summary

Adds environment variable support to all CLI flags so users don't need
to type credentials on every command.

Usage

export DUST_RPC_USER=user
export DUST_RPC_PASS=pass

# No credentials needed anymore
dust-cleaner scan
dust-cleaner sweep --dry-run
dust-cleaner sweep --method op-return

CLI flags still override env vars when explicitly provided.

Changes

• Added env feature to clap in Cargo.toml
• Added env = "DUST_RPC_*" attributes to all CLI flags in src/cli.rs

[Jolah1]

LGTM