issue_write body parameter overwrites issue content on update — easily confused with adding a comment

### Describe the feature or problem you'd like to solve

When using issue_write with method: "update", passing a ody parameter **replaces the entire issue body**. This matches the REST API semantics, but LLM agents frequently misinterpret ody as "write a comment about this issue" rather than "replace the issue's entire description". The result is silent data loss — the original issue body gets wiped and replaced with a closing note.

### Proposed solution

Two possible approaches:

1. **Add a prominent warning** in the ody parameter description, e.g.: "WARNING: This replaces the entire issue body on update, not append. Use add_issue_comment to add a comment."
2. **Add a comment parameter** to issue_write as a convenience — when provided on method: "update", it posts a comment instead of modifying the body.

Option 2 reduces unnecessary round-trips (close issue + add comment = 2 calls becomes 1).

### Example prompts or workflows

1. "Close issue `#42`and explain why it was fixed" — agent should NOT touch the original body, but currently tends to overwrite it
2. "Mark issue `#17` as duplicate and leave a note" — agent replaces body with "duplicate of #X" instead of commenting
3. "Update issue `#8` status to closed with resolution" — body gets silently destroyed

### Additional context

This was encountered using **OpenCode** (CLI-based AI coding tool) with **DeepSeek V4 Flash** model, connecting via the GitHub MCP Server (local binary). The ody field name is especially misleading for LLMs because in natural language, "body of a message" typically means the message content itself, not the metadata/description of the thing being acted upon.