LLM outputs backslashes instead of backticks in create_pull_request body, causing path prompts and control character corruption

## Description

When creating pull requests via the `create_pull_request` MCP tool, the PR body sometimes contains backslash-delimited code spans (`\code\`) instead of backtick-delimited spans (`` `code` ``). This causes two problems:

1. **False path detection**: The Copilot CLI interprets `\RequestReviewAsync\` etc. as Windows file paths and prompts the user to "allow paths" for strings that are not paths
2. **Control character corruption**: Backslash sequences that happen to match C-style escapes get interpreted as control characters:
- `\a` (e.g. `\auto_execute`) → U+0007 (bell)
- `\r` (e.g. `\request_review`) → carriage return → newline
- `\t` (e.g. `\task_complete`) → tab (U+0009)
- `\v` (e.g. `\volatile`) → vertical tab (U+000B)
- `\n` → newline (U+000A)
- `\b` → backspace (U+0008)

## Reproduction

This is intermittent — some PRs are clean while others are affected. Examples from https://github.com/m-nash/pr-copilot:

- **PR #34** (affected): Body has `\RequestReviewAsync\` instead of backtick-delimited, `\auto_execute` where `\a` became bell character (U+0007)
- **PR #33** (clean): Same repo, same user, backticks preserved correctly
- **PR #32** (affected): `\[Heartbeat]\`, `\Task.Run\`, `\volatile bool IsLoading\` where `\v` became vertical tab (U+000B)
- **PR #30** (affected): `\ask_user\` where `\a` became bell (U+0007)

## Analysis

1. The GitHub MCP server’s `CreatePullRequest` handler is a clean pass-through — it takes the `body` parameter and sends it directly to `github.NewPullRequest.Body` with no transformation
2. The corruption originates upstream, likely either:
- The LLM itself generates `\code\` instead of `` `code` `` in the JSON tool call arguments (intermittently)
- Or something in the CLI’s JSON parsing/argument extraction converts backticks to backslashes
3. The C-style escape interpretation (`\a`→bell, `\v`→vtab) suggests a non-standard string processing step, since these are NOT valid JSON escape sequences (`\a` and `\v` are C escapes, not JSON)

## Evidence that the LLM generates backslashes

When the issue occurs, the user is prompted to "allow paths" for the backslash-delimited strings in the body argument. This confirms the `\` characters are present in the tool call arguments before they reach the MCP server.

## Expected behavior

- PR bodies should preserve backtick-delimited code spans as-is
- Strings like `\RequestReviewAsync\` in tool arguments should not trigger path access prompts
- No C-style escape interpretation should be applied to PR body text

## Environment

- Copilot CLI on Windows
- GitHub MCP server (latest)
- Affects `create_pull_request` tool body parameter