Agent Skills: create-epic-pr

Create an aggregated Epic PR from a `cc-epic-<Issue number>` branch to the default branch. Takes the Epic Issue number as argument, assumes sub-PRs have already been merged into the epic branch, and automatically generates the PR title and description from the commit log against the base branch (including sub-PR / sub-Issue references). The PR is posted via `gh pr create` with no label and `Closes #<Epic Issue>`.