feat(cli): add file upload/download commands for issues

Description

Add CLI commands to upload and download files/images via the Storage API, so users can attach them to issues (e.g. paste markdown links into issue descriptions or comments).

New commands

  • pgai issues files upload <path> — upload a file, get URL and markdown link
  • pgai issues files download <url> [-o <output>] — download a file from storage

Storage API

  • POST /upload (multipart/form-data) — upload
  • GET /files/:orgId/:filename — download
  • Auth: access-token header (same API token as other commands)
  • Default base URL: https://postgres.ai/storage (configurable via --storage-base-url or PGAI_STORAGE_BASE_URL)

Acceptance criteria

  • pgai issues files upload ./screenshot.png uploads and prints URL + markdown
  • pgai issues files download /files/123/xxx.png -o ./out.png downloads the file
  • --debug and --json flags work on upload
  • Unit and integration tests cover the new functionality

Definition of Done

  • Upload and download commands merged and released in a new CLI version
  • pgai issues files upload returns a valid URL and markdown-formatted link
  • pgai issues files download correctly saves files to the specified output path
  • --debug and --json flags produce expected output for upload operations
  • Unit and integration tests pass in CI
  • Storage API base URL is configurable via --storage-base-url or PGAI_STORAGE_BASE_URL environment variable
Edited by Nikolay Samokhvalov