warn that issue_write body REPLACES content, not appends (fixes #2410)

[jluocsa]

Summary

Fixes #2410.

The body parameter on issue_write (method='update') replaces the entire issue body — matching REST API semantics, but LLM agents frequently misinterpret body as "add a note about this issue" and end up silently destroying the original issue description. Real-world examples from the linked issue:

• "Close issue [docker] build arm64 #42 and explain why it was fixed" → agent overwrites the original body with a closing note
• "Mark issue add instructions on how to test the server with VS Code #17 as duplicate and leave a note" → agent replaces body with "duplicate of #X"
• "Update issue fix: added reason argument to get_me function #8 status to closed with resolution" → body silently destroyed

This PR updates the body parameter description to make the replace-not-append semantics explicit and to point users at add_issue_comment for the common "leave a note" use case.

This is option 1 from the issue (description-only fix). Option 2 (a separate comment parameter) would be a larger schema change; happy to follow up with that as a separate PR if desired.

Before

Issue body content

After

Issue body content (Markdown). On method='update', this REPLACES the entire issue body, it does NOT append. To add a comment to an issue without modifying its body, use the add_issue_comment tool instead.

Files changed (3 files / 3 lines)

• pkg/github/issues.go — the body parameter description
• pkg/github/__toolsnaps__/issue_write.snap — regenerated via UPDATE_TOOLSNAPS=true go test ./pkg/github -run ^Test_CreateIssue$
• README.md — regenerated via go run ./cmd/github-mcp-server generate-docs

The granular issue tools (update_issue_body, add_issue_comment) don't have this confusion and were not modified.

Validation

• Test_CreateIssue passes
• go test ./... passes for all 22 packages
• Toolsnap diff is exactly the description string change
• README diff is exactly the description string change

Note: I was unable to run script/lint locally (no golangci-lint available in my environment). CI should validate it. Happy to iterate if anything surfaces.