improvement: improve diffr onboarding guidance

diffr/src/init.rs

@@ -443,6 +443,17 @@ fn prompt(state: &State) -> Result<i32> {
         }
         (Err(error), None) => return Err(error),
     }
-    cliclack::outro("diffr is ready to use! Hope you enjoy some beautiful diffs")?;
+    let usage: String = answers
+        .agents
+        .iter()
+        .map(|agent| format!("{}\n", agent.usage()))
+        .collect();
+    cliclack::outro(format!(
+        "diffr is ready to use!
+Run `diffr <args>` with the same arguments you'd pass to `git diff`, e.g. `diffr main..HEAD`.
+To change diffr settings at any time, run `diffr config`.
+{usage}
+Hope you enjoy some beautiful diffs."
+    ))?;
     Ok(0)
 }

diffr/src/install.rs

@@ -58,6 +58,15 @@ impl Agent {
         }
     }
 
+    /// How to reach diffr from inside the agent, shown once its plugin is installed.
+    pub(crate) fn usage(self) -> &'static str {
+        match self {
+            Agent::ClaudeCode => {
+                "In Claude Code, run `/diffr <args>`, or ask Claude to show you a change with diffr."
+            }
+        }
+    }
+
     /// `step` hears what each slow command is about to do.
     pub(crate) fn install(self, step: &mut dyn FnMut(&str)) -> Result<()> {
         match self {