Support custom tag message from CLI or file (for artifact metadata)

[a-magdy]

Summary

• Allow customizing the annotated tag message used by git-artifact from either a command-line string or a file/STDIN.
• This lets users embed build/CI metadata so artifacts can be discovered and audited later.

Current behavior

• Tag message content is not directly configurable from the CLI (users cannot supply an arbitrary message body).

Expected behavior / Interface

• --message, -m "": set the tag message directly from the command line (supports multi-line via shell quoting).
• --message-file, -F : read the tag message from a file. Use - to read from STDIN.
• Precedence and validation:
  • Disallow using --message and --message-file together (match git's behavior: error out if both are provided).
  • If neither flag is provided, keep the current default message behavior.
• Optional CI env support:
  • Respect env vars GIT_ARTIFACT_MESSAGE or GIT_ARTIFACT_MESSAGE_FILE when flags are not provided (useful in CI).

Notes and edge cases

• Multi-line messages should be preserved as-is (like git tag -a -m/-F).
• Very large messages: consider a reasonable size limit to avoid accidental bloat.
• Non-UTF-8 input: either reject with a clear error or document how it will be handled.
• Empty message (0 bytes): fall back to the default message or require an explicit --allow-empty-message flag.

Examples

• git-artifact create -m "Build $GITHUB_RUN_ID | SHA: $GITHUB_SHA"
• git-artifact create -F artifact-message.txt
• printf "build: %s\nurl: %s\n" "$CI_PIPELINE_ID" "$CI_JOB_URL" | git-artifact create -F -

Rationale

• Users often need to annotate artifacts with provenance metadata (pipeline ID, commit, URL, versions). First-class support makes this safer and more ergonomic.

Thanks!