Feature Request: Tab autocomplete for slash commands

Feature Request

Description:
Add tab autocomplete/completion for slash commands in the interactive input.

Current behavior:

• File path autocomplete exists (via @)
• Command history navigation works (↑/↓)
• But typing / and pressing Tab doesn't suggest available slash commands

Desired behavior:
When typing / followed by Tab (or partial command + Tab), show autocomplete suggestions for:

• Built-in slash commands (/help, /clear, /compact, etc.)
• Custom user-defined slash commands (skills)
• Fuzzy matching for partial input (e.g., /ult → /ultrathink)

Use case:
Users with many custom slash commands (skills) would benefit greatly from discoverability without needing to run /help or remember exact command names.

Example:

> /com<Tab>
  /commit
  /compact
  /compare

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