GitHub CLI: Image upload and attach for issues, PRs, and comments [GA]

### Value Prop
You can now attach local images and videos directly to issues, pull requests, and comments using the GitHub CLI. This works across `gh issue create`, `gh issue comment`, `gh pr create`, `gh pr comment`, and `gh pr edit`.

### Expected Outcome
Developers and teams using GitHub CLI in automated or agentic workflows gain a reliable way to include visual evidence like screenshots, recordings, and diagrams alongside their work. This closes a long-standing gap that made it difficult to provide proof of work or communicate changes clearly through the CLI alone. The result is a more complete CLI experience that fits naturally into both human and agentic development flows.

The issue identifies GitHub CLI commands including gh issue create, gh issue comment, gh pr create, gh pr comment, and gh pr edit, but names no repository files or tests. Start by locating the implementations and existing attachment handling for those commands. Done means local images and videos can be attached reliably across each listed workflow.

索引モデルが issue の本文から書いたものです。