[cli] clap CLI with --config flag and listen subcommand

[obchain]

Context

The first runnable binary. charon loads .env (via dotenvy), initializes structured logging, parses CLI args via clap, and loads + validates the config. This is the shell every later subcommand plugs into.

Scope

• charon-cli/src/main.rs with #[tokio::main]
• clap derive-based CLI: --config / -c flag + listen subcommand
• dotenvy::dotenv() loads .env silently if present
• tracing_subscriber with EnvFilter (defaults to INFO, overridable via RUST_LOG)
• Loads config via Config::load() and logs a one-line summary on success

Acceptance criteria

• cargo run -- --config config/default.toml listen works
• cargo run -- --help prints a usage summary
• cargo run -- --version prints the version from Cargo.toml
• Log output is structured and respects RUST_LOG
• Errors bubble up with file-path context when config is missing or malformed

References

• Delivered in commit 9a5b67a
• PRD section 3c (CLI bootstrap)

[obchain]

Superseded — clap CLI shipped in feat/05-clap-cli.