[Bug] Custom Slash Command: permission-mode Not Activating Plan Mode

Bug Description
Title:
permission-mode: plan in custom slash command frontmatter doesn't activate plan mode

Description:
### Issue
When defining custom slash commands with permission-mode: plan in the frontmatter, the plan mode indicator doesn't activate in the bottom left when executing the command.

### Example Command
.claude/commands/typecheck.md:
```yaml
---
description: Run typecheck and ultrathink a fix plan
allowed-tools: Bash(bun run typecheck), Bash(turbo), Read, Grep, Glob
permission-mode: plan
---

Expected Behavior

When running /typecheck, the plan mode indicator should appear in the bottom left corner.

Actual Behavior

Plan mode doesn't activate. The command runs in normal mode.

Questions

1. Is permission-mode a supported frontmatter option for custom slash commands?
2. If not, could this feature be added to allow commands to auto-activate plan mode?
3. If it is supported, why isn't it working?

Current Workaround

Manually pressing Shift+Tab before running the command.
```

Environment Info

• Platform: darwin
• Terminal: ghostty
• Version: 2.0.0
• Feedback ID: 30d3dd73-e9fe-4458-aa60-1fac55b79e50

Errors

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