[FEATURE] Subcommand autocomplete for custom slash commands

Feature Request

Problem

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

For example, /contextr has 14 subcommands (add, list, remove, clear, checkpoint, transcript, init, scrub, promote, analyze, weekly, review-period, config, help). After typing /contextr , the user gets no suggestions — they must remember the subcommand or type /contextr help first.

Proposed Solution

Allow skill frontmatter to declare subcommands:

---
description: Context persistence manager
subcommands:
  - add [--sticky] <text>
  - list
  - remove <number>
  - clear [--all]
  - checkpoint
  - transcript list|review|search
  - analyze
  - weekly
  - config projects-dir|secret-store-vault
---

After the user selects the top-level command, the TUI would show these as completions.

Why This Matters

Skills that manage complex workflows naturally grow subcommands. Without autocomplete, users either:

• Memorize the subcommand list (poor DX)
• Type help every time (wastes tokens)
• Split into N separate skills (/contextr-add, /contextr-weekly...) which pollutes the command namespace

Prior Issues

This has been requested multiple times and closed as stale each time:

• #28195 — "[FEATURE] Support subcommand autocomplete for custom slash commands" (April 2026, 3 comments)
• #23832 — "Feature: Skill subcommand autocomplete support" (February 2026)
• #56918 — "Dynamic autosuggestions for slash command arguments" (June 2026)

The repeated requests suggest real demand from the skill-authoring community.

Alternatives Considered

• Split into separate skills: Works but clutters the / menu with dozens of entries
• Argument hints: #59644 shows argument-hint exists but only renders static text, not interactive completions
• Plugin subcommands: #63772 notes the same gap for plugins