add but squash command with AI message support

crates/but/src/args/metrics.rs

@@ -9,6 +9,7 @@ pub enum CommandName {
     Stf,
     Rub,
     Diff,
+    Squash,
     Commit,
     Push,
     New,

crates/but/src/args/mod.rs

@@ -419,6 +419,40 @@ pub enum Subcommands {
         force: bool,
     },
 
+    /// Squash multiple commits together and use AI to generate a combined commit message.
+    ///
+    /// This command takes two or more commit IDs or CLI short IDs. All commits must be
+    /// in the same stack and form a contiguous range. The commits are squashed together
+    /// into the oldest (parent-most) commit, and AI is used to generate a new combined
+    /// commit message from all original messages.
+    ///
+    /// ## Examples
+    ///
+    /// Squash two commits using their SHAs:
+    ///
+    /// ```text
+    /// but squash abc1234 def5678
+    /// ```
+    ///
+    /// Squash three commits using CLI short IDs from `but status`:
+    ///
+    /// ```text
+    /// but squash 3a 4b 5c
+    /// ```
+    ///
+    /// Squash a range of commits:
+    ///
+    /// ```text
+    /// but squash 2a 3a 4a 5a
+    /// ```
+    ///
+    #[cfg(feature = "legacy")]
+    Squash {
+        /// The commit IDs or CLI short IDs to squash together (minimum 2)
+        #[clap(required = true, num_args = 2..)]
+        commits: Vec<String>,
+    },
+
     /// Undo the last operation by reverting to the previous snapshot.
     ///
     /// This is a shorthand for restoring to the last oplog entry before the

crates/but/src/command/help.rs

@@ -40,7 +40,7 @@ pub fn print_grouped(out: &mut dyn std::fmt::Write) -> std::fmt::Result {
         ("Server Interactions".yellow(), vec!["push", "pr", "forge"]),
         (
             "Editing Commits".yellow(),
-            vec!["rub", "describe", "absorb"],
+            vec!["rub", "squash", "describe", "absorb"],
         ),
         (
             "Operation History".yellow(),

crates/but/src/command/legacy/mod.rs

@@ -17,5 +17,6 @@ pub mod oplog;
 pub mod pull;
 pub mod push;
 pub mod rub;
+pub mod squash;
 pub mod status;
 pub mod worktree;