test: add offline agent end-to-end coverage

[9Ashwin]

What changed

• extend scripts/agent-e2e.py into an offline, temporary-home acceptance flow for local curated setup, full-pack exclusion, offline search/get, WebP preview, animated GIF bytes, v1 import, favorites collections, four sort modes, dry-run/batch operations, and final manifest/collection ordering
• assert stable JSON errors and no publication after bad image hashes, corrupt collection metadata, invalid batch IDs, and rejected release archive entries
• run the e2e job on Linux and macOS with CGO disabled; keep the release installer’s base URL override for the local HTTPS archive fixture
• document the CI/e2e coverage

Closes #20