fix(shell): expand command subs in nested contexts and assignments

- Fix expand_command_substitution to use expand_with_command_sub for args (supports nested command subs)
- Fix Assignment to use expand_with_command_sub (supports VAR=$(cmd))
- Fix Export to use expand_with_command_sub (supports export VAR=$(cmd))

Now working correctly:
- Nested command subs: echo $(outer $(inner)) → "outer inner"
- Variable assignment: VAR=$(pwd); echo $VAR → correct path
- Export with command sub: export PATH=$(pwd):$PATH

All 128 tests still passing.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: hyperpolymath

impl/rust-cli/Cargo.lock

@@ -349,8 +349,8 @@ impl ExecutableCommand for Command {
 
             // Variable assignment
             Command::Assignment { name, value } => {
-                // Expand variables in the value
-                let expanded_value = crate::parser::expand_variables(value, state);
+                // Expand variables and command substitutions in the value
+                let expanded_value = crate::parser::expand_with_command_sub(value, state)?;
                 state.set_variable(name, expanded_value);
                 Ok(ExecutionResult::Success)
             }
@@ -359,8 +359,8 @@ impl ExecutableCommand for Command {
             Command::Export { name, value } => {
                 if let Some(val) = value {
                     // export VAR=value
-                    // Expand variables in the value
-                    let expanded_value = crate::parser::expand_variables(val, state);
+                    // Expand variables and command substitutions in the value
+                    let expanded_value = crate::parser::expand_with_command_sub(val, state)?;
                     state.set_variable(name, expanded_value);
                     state.export_variable(name);
                 } else {

impl/rust-cli/src/executable.rs

@@ -1146,11 +1146,12 @@ pub fn expand_command_substitution(cmd: &str, state: &mut crate::state::ShellSta
             process_cmd.stdout(Stdio::piped());
             process_cmd.stderr(Stdio::null());
 
-            // Expand variables in args before execution
-            let expanded_args: Vec<String> = args
+            // Expand variables and command substitutions in args before execution
+            let expanded_args: Result<Vec<String>> = args
                 .iter()
-                .map(|arg| expand_variables(arg, state))
+                .map(|arg| expand_with_command_sub(arg, state))
                 .collect();
+            let expanded_args = expanded_args?;
 
             process_cmd.args(&expanded_args);