feat: add bash-style array slicing for $@ in prompt templates

Implements support for ${@:N} and ${@:N:L} syntax to slice argument arrays
in prompt templates, following bash conventions.

Syntax:
- ${@:N} - All arguments from Nth position onwards (1-indexed)
- ${@:N:L} - L arguments starting from Nth position

Features:
- Bash-style slicing familiar to shell users
- 1-indexed for consistency with $1, $2, etc.
- Processes before simple $@ to avoid conflicts
- No recursive substitution of patterns in arguments
- Comprehensive edge case handling

Examples:
- ${@:2} with ["a", "b", "c"] -> "b c"
- ${@:2:1} with ["a", "b", "c"] -> "b"
- ${@:99} with ["a", "b"] -> "" (empty, out of range)

Test coverage: 24 new tests, all passing (73 total)

Closes #769

packages/coding-agent/src/core/prompt-templates.ts

@@ -52,7 +52,11 @@ export function parseCommandArgs(argsString: string): string[] {
 
 /**
  * Substitute argument placeholders in template content
- * Supports $1, $2, ... for positional args, $@ and $ARGUMENTS for all args
+ * Supports:
+ * - $1, $2, ... for positional args
+ * - $@ and $ARGUMENTS for all args
+ * - ${@:N} for args from Nth onwards (bash-style slicing)
+ * - ${@:N:L} for L args starting from Nth
  *
  * Note: Replacement happens on the template string only. Argument values
  * containing patterns like $1, $@, or $ARGUMENTS are NOT recursively substituted.
@@ -67,6 +71,20 @@ export function substituteArgs(content: string, args: string[]): string {
 		return args[index] ?? "";
 	});
 
+	// Replace ${@:start} or ${@:start:length} with sliced args (bash-style)
+	// Process BEFORE simple $@ to avoid conflicts
+	result = result.replace(/\$\{@:(\d+)(?::(\d+))?\}/g, (_, startStr, lengthStr) => {
+		let start = parseInt(startStr, 10) - 1; // Convert to 0-indexed (user provides 1-indexed)
+		// Treat 0 as 1 (bash convention: args start at 1)
+		if (start < 0) start = 0;
+
+		if (lengthStr) {
+			const length = parseInt(lengthStr, 10);
+			return args.slice(start, start + length).join(" ");
+		}
+		return args.slice(start).join(" ");
+	});
+
 	// Pre-compute all args joined (optimization)
 	const allArgs = args.join(" ");