Improve error messages for missing stack commits

Clarify why a commit might not be found and suggest running 'but status'
to refresh state. 

Provide more context in errors for source/target lookups, and include 
the short commit id in squash/undo errors so users know which commit is 
affected.

Author: schacon

crates/but/src/rub/mod.rs

@@ -112,7 +112,10 @@ fn ids(ctx: &mut CommandContext, source: &str, target: &str) -> anyhow::Result<(
     let source_result = crate::id::CliId::from_str(ctx, source)?;
     if source_result.len() != 1 {
         if source_result.is_empty() {
-            return Err(anyhow::anyhow!("Source '{}' not found", source));
+            return Err(anyhow::anyhow!(
+                "Source '{}' not found. If you just performed a Git operation (squash, rebase, etc.), try running 'but status' to refresh the current state.", 
+                source
+            ));
         } else {
             let matches: Vec<String> = source_result.iter().map(|id| {
                 match id {
@@ -131,7 +134,10 @@ fn ids(ctx: &mut CommandContext, source: &str, target: &str) -> anyhow::Result<(
     let target_result = crate::id::CliId::from_str(ctx, target)?;
     if target_result.len() != 1 {
         if target_result.is_empty() {
-            return Err(anyhow::anyhow!("Target '{}' not found", target));
+            return Err(anyhow::anyhow!(
+                "Target '{}' not found. If you just performed a Git operation (squash, rebase, etc.), try running 'but status' to refresh the current state.", 
+                target
+            ));
         } else {
             let matches: Vec<String> = target_result.iter().map(|id| {
                 match id {

crates/but/src/rub/squash.rs

@@ -10,8 +10,13 @@ pub(crate) fn commits(
     source: &ObjectId,
     destination: &ObjectId,
 ) -> anyhow::Result<()> {
-    let source_stack = stack_id_by_commit_id(ctx, source)?;
-    let destination_stack = stack_id_by_commit_id(ctx, destination)?;
+    // Validate both commits exist in stacks before proceeding
+    let source_stack = stack_id_by_commit_id(ctx, source).map_err(|e| {
+        anyhow::anyhow!("Source commit {}: {}", &source.to_string()[..7], e)
+    })?;
+    let destination_stack = stack_id_by_commit_id(ctx, destination).map_err(|e| {
+        anyhow::anyhow!("Destination commit {}: {}", &destination.to_string()[..7], e)
+    })?;
     if source_stack != destination_stack {
         anyhow::bail!("Cannot squash commits from different stacks");
     }

crates/but/src/rub/undo.rs

@@ -29,5 +29,8 @@ pub(crate) fn stack_id_by_commit_id(
     }) {
         return Ok(*id);
     }
-    anyhow::bail!("No stack found for commit {}", oid.to_string())
+    anyhow::bail!(
+        "No stack found for commit {}. The commit may have been removed by a previous operation (e.g., squash, rebase). Try refreshing with 'but status' to see the current state.", 
+        &oid.to_string()[..7]
+    )
 }