feat: add setup did command to publish labeler identity

`cargo run --bin setup -- did` adds #atproto_label and #atproto_labeler to the
DID document via the PDS-signed PLC flow, prompting for the main password and
the emailed code. Splits setup into declare/did modules.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/bin/setup/main.rs

@@ -0,0 +1,19 @@
+//! One-time labeler setup.
+//!
+//! - `cargo run --bin setup -- declare` publishes the label definitions.
+//! - `cargo run --bin setup -- did` publishes the labeler's identity entries.
+mod declare;
+mod did;
+
+use anyhow::{Result, bail};
+use cities_labeler::config::Config;
+
+#[tokio::main]
+async fn main() -> Result<()> {
+    let config = Config::from_env()?;
+    match std::env::args().nth(1).as_deref() {
+        Some("declare") => declare::run(&config).await,
+        Some("did") => did::run(&config).await,
+        _ => bail!("usage: setup <declare|did>"),
+    }
+}