Feature Request: Expose session_name in hook payloads and add Command hook event

Summary

Hook payloads currently include session_id but not the human-readable session_name that can be set via /rename. For registry/monitoring use cases, having the session name would be valuable.

Additionally, there's no hook event for slash commands (like /rename), which would enable workflows that need to intercept or respond to user commands.

Use Case

I'm building a session registry that tracks active Claude Code instances. Currently it can only display the cwd path, which isn't very descriptive. If session_name were available in hook payloads (particularly SessionStart), the registry could display meaningful names.

Even better, if there were a Command hook event (or PreCommand/PostCommand), I could:

1. Intercept /rename <name> commands
2. Sync the name to my external registry
3. Keep both systems in sync without manual duplication

Requested Changes

1. Add session_name to hook payloads

Include the session name (if set) in relevant hook payloads:

{
  "session_id": "abc123",
  "session_name": "auth-refactor",  // NEW - null if not set
  "transcript_path": "...",
  ...
}

At minimum in SessionStart, but ideally in all hooks.

2. (Optional) Add Command hook event

A hook that fires when slash commands are executed:

{
  "hooks": {
    "Command": [
      {
        "matcher": "rename",
        "hooks": [{ "type": "command", "command": "./on-rename.sh" }]
      }
    ]
  }
}

Input:

{
  "session_id": "abc123",
  "hook_event_name": "Command",
  "command_name": "rename",
  "command_args": "auth-refactor"
}

This would enable intercepting/extending built-in commands for custom workflows.

Current Workaround

Without these features, session registries must either:

• Display only paths (less descriptive)
• Maintain a separate naming system (risk of desync)
• Parse transcript files directly (fragile, not a stable API)

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Thank you for considering this feature request!