warn that issue_write body REPLACES content, not appends

The `body` parameter on `issue_write` (method='update') replaces the
entire issue body, matching REST API semantics. However, LLM agents
frequently misinterpret `body` as "add a note about this issue" and
end up silently destroying the original issue description (#2410).

Update the 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.

Updates the issue_write toolsnap and README accordingly.

Fixes #2410.

Author: jluocsa

README.md

@@ -853,7 +853,7 @@ The following sets of tools are available:
 - **issue_write** - Create or update issue
   - **Required OAuth Scopes**: `repo`
   - `assignees`: Usernames to assign to this issue (string[], optional)
-  - `body`: Issue body content (string, optional)
+  - `body`: 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. (string, optional)
   - `duplicate_of`: Issue number that this issue is a duplicate of. Only used when state_reason is 'duplicate'. (number, optional)
   - `issue_number`: Issue number to update (number, optional)
   - `labels`: Labels to apply to this issue (string[], optional)

pkg/github/__toolsnaps__/issue_write.snap

@@ -22,7 +22,7 @@
         "type": "array"
       },
       "body": {
-        "description": "Issue body content",
+        "description": "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.",
         "type": "string"
       },
       "duplicate_of": {

pkg/github/issues.go

@@ -1471,7 +1471,7 @@ Options are:
 					},
 					"body": {
 						Type:        "string",
-						Description: "Issue body content",
+						Description: "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.",
 					},
 					"assignees": {
 						Type:        "array",