Add /sub-issue and /parent-issue comment commands for linking sub-issues from either end

[Ma77Ball]

Feature Summary

GitHub supports sub-issue relationships, but linking and unlinking them today requires clicking through the issue UI; there's no way to do it from a comment. Contributors triaging issues often have the relationship in mind while writing a comment and would rather express it in-line, the same way they self-claim issues with /take.

This adds a small family of comment commands that let you create or remove a sub-issue link on either side of the relationship without leaving the comment box.

Proposed Solution or Design

Extend .github/workflows/comment-commands.yml with a new sub-issue job that recognizes four commands on issue comments (not PRs):

Comment on	Command	Effect
parent	/sub-issue #N [#M ...]	Links #N (and any further numbers) as sub-issues of the current issue
parent	/unsub-issue #N [#M ...]	Unlinks #N from the current issue's sub-issues
child	/parent-issue #N	Sets #N as the parent of the current issue
child	/unparent-issue (or /unparent-issue #N)	Removes the current issue from its parent; with no argument, the parent is resolved via GraphQL

Example, from a parent issue's comment box:

/sub-issue #1234 #1235

Example, from a child issue:

/parent-issue #1200

Affected Area

Other

[Ma77Ball]

/take