Add `list` subcommand to enumerate release assets without downloading

[fuleinist]

Problem

dra download <repo> either prompts interactively (TTY) or requires the user to know a pattern (--select) or rely on automatic matching (--automatic). When scripting around dra — e.g. a setup script that wants to detect whether a repo ships an aarch64 build, a .deb package, or a portable tarball — there's no way to inspect what assets exist without entering the interactive picker or attempting a download. Pipelines that need to fail fast on missing assets can't detect the missing-asset case ahead of time.

Solution

Add a dra list <repo> subcommand that reuses the existing fetch_release_for + Release.assets path (already used by UntagHandler::run) and prints the available assets to stdout. Two output modes:

• Default (human): one line per asset in name | display_name | size order, e.g. hello_1.0.0_amd64.deb | | 12.4MB.
• --json: machine-readable, e.g. [{name: hello_1.0.0_amd64.deb, display_name: null, size_bytes: 13002342, download_url: ...}, ...]. Reuses the serde Serialize derive pattern from the Asset struct.

Optional --tag <v> flag to inspect a specific release (parallels the existing download --tag, reuses fetch_release_for(..., Some(tag))).

Use case

# Detect whether a repo ships an aarch64 Linux build before running setup
if dra list --json owner/repo | jq -e '.[] | select(.name | test("aarch64|arm64"))' >/dev/null; then
  dra download --select '<aarch64 asset>' owner/repo
else
  echo 'no aarch64 build available' >&2
  exit 1
fi

Today, the only option is to either guess the asset name (and have dra fail with a pattern-not-matched error after the network call) or run dra download interactively and capture stdout — neither fits a non-interactive setup script.

Distinct from existing

• dra download --select <pattern> — requires a pattern, doesn't enumerate.
• dra untag <repo> — interactive picker, then prints a single untagged name; doesn't expose the full asset list.
• dra download --automatic — picks a single best match silently; doesn't expose the choices.

Disclosure

Filing from fuleinist, a recurring GitHub-issue contributor. Happy to send a PR if the maintainer is interested — the implementation should be ~50-80 lines (src/cli/list_handler.rs + a List variant on Command + clap derive for --tag/--json).