Asking Dependabot to Rebase via MCP leads to malformed text in comment

### Describe the bug

`add_issue_comment` mangles @mentions in comment bodies by inserting Unicode middle dot characters (·) between letters. This causes bot commands (e.g. Dependabot's `@dependabot rebase`) to silently fail because the mention is unrecognized.

### Affected version

Version unknown; observed on June 16, 2026

### Steps to reproduce the behavior

1. In Claude Code, ask Claude to post `@dependabot rebase` as a comment on a GitHub PR via the `add_issue_comment` MCP tool.
2. Observe the resulting comment body on GitHub.

### Expected behavior
Comment body: `@dependabot rebase`

### Actual behavior

Comment body: `·@·d·ependabot r·ebase`

### Additional context

The issue reproduces consistently on repeated calls with the same input. The mangled text prevents Dependabot from recognizing the command.