feat: display name in banner + prompt

Banner shows human-readable principal and DID on separate lines:
  Principal: tking@guildhouse.dev
  DID:       did:web:guildhouse.dev/user/tking

Prompt uses short name: [governed] tking@gsh

Reads BASCULE_DISPLAY_NAME env. Fallback: parse DID to name@domain.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

gsh/src/human.rs

@@ -150,7 +150,8 @@ fn print_banner(session: &SessionState) {
     println!();
     println!("{}", "╔══════════════════════════════════════════════════════════╗".bright_blue());
     println!("{}  Guildhouse Governed Shell v0.1.0{}", "║".bright_blue(), " ".repeat(24).to_string() + &"║".bright_blue().to_string());
-    println!("{}  Principal: {:<44}{}", "║".bright_blue(), session.principal, "║".bright_blue());
+    println!("{}  Principal: {:<44}{}", "║".bright_blue(), session.display_name, "║".bright_blue());
+    println!("{}  DID:       {:<44}{}", "║".bright_blue(), session.principal, "║".bright_blue());
     println!("{}  Corpus:    {:<44}{}", "║".bright_blue(), corpus_short, "║".bright_blue());
     println!("{}  Session:   {:<44}{}", "║".bright_blue(), format!("{} (expires {})", &session.ac_id[..8.min(session.ac_id.len())], expiry), "║".bright_blue());
     println!("{}  Risk:      {:<44}{}", "║".bright_blue(),
@@ -199,10 +200,10 @@ fn build_prompt(session: &SessionState) -> DefaultPrompt {
         _ => "[governed]",
     };
 
-    let user = session.principal.split('@').next().unwrap_or(&session.principal);
+    let short_name = session.display_name.split('@').next().unwrap_or(&session.display_name);
 
     DefaultPrompt::new(
-        DefaultPromptSegment::Basic(format!("{} {}@gsh", risk_indicator, user)),
+        DefaultPromptSegment::Basic(format!("{} {}@gsh", risk_indicator, short_name)),
         DefaultPromptSegment::Empty,
     )
 }

libgsh/src/session.rs

@@ -7,6 +7,7 @@ pub struct SessionState {
     pub ac_id: String,
     pub corpus_cid: String,
     pub principal: String,
+    pub display_name: String,
     pub risk_level: String,
     pub started_at: chrono::DateTime<chrono::Utc>,
     pub expires_at: Option<chrono::DateTime<chrono::Utc>>,
@@ -35,10 +36,15 @@ impl SessionState {
             .and_then(|s| chrono::DateTime::parse_from_rfc3339(s).ok())
             .map(|dt| dt.with_timezone(&chrono::Utc));
 
+        // Display name: BASCULE_DISPLAY_NAME env, or derive from DID
+        let display_name = std::env::var("BASCULE_DISPLAY_NAME")
+            .unwrap_or_else(|_| display_name_from_did(&principal));
+
         Self {
             ac_id: ac.context_id.clone(),
             corpus_cid: corpus_cid.to_string(),
             principal,
+            display_name,
             risk_level: "standard".to_string(), // TODO: read from AC when broker embeds it
             started_at: chrono::Utc::now(),
             expires_at,
@@ -51,10 +57,14 @@ impl SessionState {
 
     /// Create a minimal session for ungoverned mode.
     pub fn ungoverned(corpus_cid: &str) -> Self {
+        let principal = whoami();
+        let display_name = std::env::var("BASCULE_DISPLAY_NAME")
+            .unwrap_or_else(|_| principal.clone());
         Self {
             ac_id: "ungoverned".to_string(),
             corpus_cid: corpus_cid.to_string(),
-            principal: whoami(),
+            principal,
+            display_name,
             risk_level: "ungoverned".to_string(),
             started_at: chrono::Utc::now(),
             expires_at: None,
@@ -82,3 +92,18 @@ fn whoami() -> String {
         .or_else(|_| std::env::var("USERNAME"))
         .unwrap_or_else(|_| "operator".to_string())
 }
+
+/// Derive a human-readable display name from a DID.
+/// did:web:guildhouse.dev/user/tking → tking@guildhouse.dev
+/// Fallback: return the full DID.
+fn display_name_from_did(did: &str) -> String {
+    if let Some(rest) = did.strip_prefix("did:web:") {
+        let parts: Vec<&str> = rest.splitn(2, '/').collect();
+        if parts.len() == 2 {
+            let domain = parts[0];
+            let name = parts[1].rsplit('/').next().unwrap_or(parts[1]);
+            return format!("{}@{}", name, domain);
+        }
+    }
+    did.to_string()
+}