Summary:
The create-issue skill hardcoded `--assignee @me` on every `gh issue
create`, so any issue it filed was always assigned to whoever invoked
the skill, with no way to leave it unassigned or assign someone else.

This change:
- create-issue now asks whether the user has someone in mind to assign
  the issue to, and otherwise leaves it unassigned. Applied to both the
  GitHub (Step 3a) and JIRA (Step 3b) paths, with step numbering fixed.
- create-pr and create-diff auto-create an issue that tracks the current
  work, so they now instruct create-issue to assign that issue to the
  invoker (`@me`).

Test Plan:
Documentation-only change to agent skill instructions (Markdown under
.claude/skills/); no code paths or automated tests are affected.
- ./build-support/lint.sh: 0 errors, 0 warnings.
- Reviewed the rendered skill instructions for consistent step
  numbering and correct cross-references between create-pr/create-diff
  and create-issue.

Reviewers: hsunder

Reviewed By: hsunder

Subscribers: ybase

Differential Revision: https://phorge.dev.yugabyte.com/D55944