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/src/main.rs

@@ -14,6 +14,7 @@ enum Cmd {
     Write { path: String, content: String },
     Edit { path: String, old_string: String, new_string: String },
     Bash { command: String, #[arg(long)] timeout: Option<u64> },
+    Slash { command_name: String, args: Vec<String> },
 }
 
 #[derive(Parser, Debug)]
@@ -180,6 +181,47 @@ async fn main() -> Result<()> {
                     }
                 }
             }
+            Cmd::Slash { command_name, args } => {
+                // Check permission
+                match perms.check(Tool::SlashCommand, None) {
+                    PermissionDecision::Allow => {
+                        // Look for command file in .claude/commands/
+                        let command_path = format!(".claude/commands/{}.md", command_name);
+
+                        // Read the command file
+                        let content = match tools_fs::read_file(&command_path) {
+                            Ok(c) => c,
+                            Err(_) => {
+                                return Err(eyre!(
+                                    "Slash command '{}' not found at {}",
+                                    command_name,
+                                    command_path
+                                ));
+                            }
+                        };
+
+                        // Parse with arguments
+                        let args_refs: Vec<&str> = args.iter().map(|s| s.as_str()).collect();
+                        let slash_cmd = tools_slash::parse_slash_command(&content, &args_refs)?;
+
+                        // Resolve file references
+                        let resolved_body = slash_cmd.resolve_file_refs()?;
+
+                        // Print the resolved command body
+                        println!("{}", resolved_body);
+
+                        return Ok(());
+                    }
+                    PermissionDecision::Ask => {
+                        return Err(eyre!(
+                            "Permission denied: Slash command requires approval. Use --mode code to allow."
+                        ));
+                    }
+                    PermissionDecision::Deny => {
+                        return Err(eyre!("Permission denied: Slash command is blocked."));
+                    }
+                }
+            }
         }
     }