Add tab completion support for custom slash commands

Description:
## Feature Request: Tab Completion for Custom Slash Commands

### Summary
Add tab completion support for user-defined slash commands, similar to how built-in commands like /help and /compact work.

### Current Behavior

• Built-in slash commands have tab completion
• Custom slash commands (like workflow patterns) require typing the full command
• No autocomplete or suggestion system for user-defined commands

### Proposed Feature
Allow users to define custom slash commands with tab completion in their configuration files (e.g., CLAUDE.md or settings).

### Example Use Case
I've created Linear workflow commands like:

• /linear-start-work [issue-id] - Start work on a Linear issue
• /linear-complete [issue-id] - Mark Linear issue as complete
• /linear-create [title] - Create new Linear issue

These would benefit greatly from tab completion, especially when used frequently.

### Suggested Implementation
```yaml
# In CLAUDE.md or settings
slash_commands:

• command: "linear-start-work"

description: "Start work on a Linear issue"
parameters: ["issue-id"]

• command: "linear-complete"

description: "Mark Linear issue as complete"
parameters: ["issue-id"]

Benefits

• Improved developer productivity
• Reduced typing errors
• Better discoverability of custom workflows
• Consistent UX with built-in commands

Current Workaround

Currently using full command typing without completion support.