[FEATURE] Support subcommand autocomplete for custom slash commands

Feature Request

Problem

Custom slash commands (skills) registered via .claude/commands/ or skill files only support top-level autocomplete in the / menu. There is no way to register subcommands or show contextual suggestions after the user selects a slash command.

For example, I have a tool called LIAM that registers /liam as a skill. It has subcommands like review, pr create, commit-push, feedback, init, doctor, etc. When a user types /liam , they get no suggestions for available subcommands. They have to know the commands or type /liam help first.

Desired Behaviour

After selecting a slash command from the / menu, show a second-level autocomplete with registered subcommands:

/liam          <- selects the skill
/liam r        <- shows: review, (filtered list)
/liam review   <- selects the subcommand, cursor moves to args

Suggested Approach

Allow skills to declare subcommands in frontmatter:

---
name: liam
description: Layered Intelligent Agent Management
argument-hint: "[command] [args...]"
subcommands:
  - name: review
    description: Code review via specialist reviewer swarm
    argument-hint: "[PR number|URL|--diff file]"
  - name: pr create
    description: Create PR with validation and Jira integration
  - name: pr update
    description: Update existing PR description
  - name: feedback
    description: Address unaddressed PR review comments
    argument-hint: "[PR number]"
  - name: implement
    description: Implement feature from task source
    argument-hint: "[ticket|URL|description]"
  - name: commit-push
    description: AI commit message, stage, commit, and push
    argument-hint: "[-y] [-a]"
  - name: init
    description: Set up project context or credentials
  - name: doctor
    description: Run health diagnostics
  - name: help
    description: Show available commands
---

Why This Matters

• Discoverability: Users shouldn't need to memorise subcommands or type help to find them
• Consistency: The gh CLI, git, and other tools with subcommands all support tab completion. Slash commands in Claude Code should too
• Ecosystem growth: As the skill/command ecosystem matures, tools will naturally develop subcommand hierarchies. Supporting this early avoids fragmentation (registering /liam-review, /liam-pr, /liam-commit as separate skills)

Current Workarounds

1. Single skill with internal routing and /liam help for discovery (what I use now)
2. Register each subcommand as a separate skill (/liam-review, /liam-pr-create), which bloats the / menu and loses the namespace

Neither is ideal.

Environment

• Claude Code CLI
• macOS / Linux