feat(tools): implement Slash Commands with frontmatter and file refs (M5 complete)

This commit implements the complete M5 milestone (Slash Commands) including:

Slash Command Parser (tools-slash):
- YAML frontmatter parsing with serde_yaml
- Metadata extraction (description, author, tags, version)
- Arbitrary frontmatter fields via flattened HashMap
- Graceful fallback for commands without frontmatter

Argument Substitution:
- $ARGUMENTS - all arguments joined by space
- $1, $2, $3, etc. - positional arguments
- Unmatched placeholders remain unchanged
- Empty arguments result in empty string for $ARGUMENTS

File Reference Resolution:
- @path syntax to include file contents inline
- Regex-based matching for file references
- Multiple file references supported
- Clear error messages for missing files

CLI Integration:
- Added `slash` subcommand: `owlen slash <command> <args...>`
- Loads commands from `.claude/commands/<name>.md`
- Permission checks for SlashCommand tool
- Automatic file reference resolution before output

Command Structure:
---
description: "Command description"
author: "Author name"
tags:
  - tag1
  - tag2
---
Command body with $ARGUMENTS and @file.txt references

Permission Enforcement:
- Plan mode: SlashCommand allowed (utility tool)
- All modes: SlashCommand respects permissions
- File references respect filesystem permissions

Testing:
- 10 tests in tools-slash for parser functionality
  - Frontmatter parsing with complex YAML
  - Argument substitution (all variants)
  - File reference resolution (single and multiple)
  - Edge cases (no frontmatter, empty args, etc.)
- 3 new tests in CLI for integration
  - slash_command_works (with args and frontmatter)
  - slash_command_file_refs (file inclusion)
  - slash_command_not_found (error handling)
- All 56 workspace tests passing ✅

Dependencies Added:
- serde_yaml 0.9 for YAML frontmatter parsing
- regex 1.12 for file reference pattern matching

M5 milestone complete! ✅

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

crates/app/cli/tests/permissions.rs

@@ -177,3 +177,79 @@ fn bash_command_timeout_works() {
         .arg("1000");
     cmd.assert().failure();
 }
+
+#[test]
+fn slash_command_works() {
+    // Create .claude/commands directory in temp dir
+    let dir = tempdir().unwrap();
+    let commands_dir = dir.path().join(".claude/commands");
+    fs::create_dir_all(&commands_dir).unwrap();
+
+    // Create a test slash command
+    let command_content = r#"---
+description: "Test command"
+---
+Hello from slash command!
+Args: $ARGUMENTS
+First: $1
+"#;
+    let command_file = commands_dir.join("test.md");
+    fs::write(&command_file, command_content).unwrap();
+
+    // Execute slash command with args from the temp directory
+    let mut cmd = Command::new(assert_cmd::cargo::cargo_bin!("owlen"));
+    cmd.current_dir(dir.path())
+        .arg("--mode")
+        .arg("code")
+        .arg("slash")
+        .arg("test")
+        .arg("arg1");
+
+    cmd.assert()
+        .success()
+        .stdout(predicates::str::contains("Hello from slash command!"))
+        .stdout(predicates::str::contains("Args: arg1"))
+        .stdout(predicates::str::contains("First: arg1"));
+}
+
+#[test]
+fn slash_command_file_refs() {
+    let dir = tempdir().unwrap();
+    let commands_dir = dir.path().join(".claude/commands");
+    fs::create_dir_all(&commands_dir).unwrap();
+
+    // Create a file to reference
+    let data_file = dir.path().join("data.txt");
+    fs::write(&data_file, "Referenced content").unwrap();
+
+    // Create slash command with file reference
+    let command_content = format!("File content: @{}", data_file.display());
+    fs::write(commands_dir.join("reftest.md"), command_content).unwrap();
+
+    // Execute slash command
+    let mut cmd = Command::new(assert_cmd::cargo::cargo_bin!("owlen"));
+    cmd.current_dir(dir.path())
+        .arg("--mode")
+        .arg("code")
+        .arg("slash")
+        .arg("reftest");
+
+    cmd.assert()
+        .success()
+        .stdout(predicates::str::contains("Referenced content"));
+}
+
+#[test]
+fn slash_command_not_found() {
+    let dir = tempdir().unwrap();
+
+    // Try to execute non-existent slash command
+    let mut cmd = Command::new(assert_cmd::cargo::cargo_bin!("owlen"));
+    cmd.current_dir(dir.path())
+        .arg("--mode")
+        .arg("code")
+        .arg("slash")
+        .arg("nonexistent");
+
+    cmd.assert().failure();
+}