fix: add explicit destructiveHint false to non-destructive write tools

[larik15]

What

Several clearly additive write tools set ReadOnlyHint: false but omit
DestructiveHint, so the MCP ToolAnnotations contract defaults them to
destructiveHint: true. Conservative clients then require extra approval for
routine operations like creating a branch or opening a pull request.

This adds an explicit DestructiveHint: jsonschema.Ptr(false) to write tools
that only create or add, following the existing pattern in the granular
toolsets.

Tools annotated

• create_gist
• create_repository
• fork_repository
• create_branch
• star_repository
• create_pull_request
• add_reply_to_pull_request_comment
• add_comment_to_pending_review
• add_issue_comment
• request_copilot_review
• create_repository_ruleset

Deliberately left conservative

Tools that can overwrite or remove existing state, or whose behaviour depends
on the requested method, keep the conservative default:

• update_gist, update_issue_comment — overwrite existing content
• create_or_update_file, push_files — can overwrite files
• merge_pull_request — irreversible
• update_pull_request_branch — can introduce conflicts
• custom_properties_write — last-write-wins on concurrent updates
• unstar_repository, dismiss_notification, mark_all_notifications_read — remove or reset state
• issue_write, update_pull_request, issue_dependency_write, sub_issue_write, pull_request_review_write — method-dependent, including destructive methods

Testing

go test ./pkg/github/... passes. Tool snapshots updated for the annotated tools.

Fixes #3281