Add --tag to release upload for parity with release create

[CloudCorpRecords]

Fixes #168

forge release create takes --tag, but forge release upload only accepted the tag positionally, which is awkward in scripts. This adds --tag to upload:

forge release upload v1.0.0 app.zip          # unchanged
forge release upload --tag v1.0.0 app.zip    # new

• With --tag, the single argument is the file; without it, the existing <tag> <file> form applies, so nothing breaks.
• Supplying --tag and a positional tag, no tag at all, or the wrong number of arguments gives a clear error.
• The argument handling lives in a small pure helper, uploadTagAndFile, so it is unit-tested directly (7 cases), plus a test that the flag exists.

I left release download <tag> <asset-name> alone since the issue is about upload; happy to give it the same --tag treatment in a follow-up if you want.

Checks: go test -race ./... and golangci-lint run pass locally.

This change was written with Claude Code (an AI coding agent); I ran the tests and linter above.

[andrew]

Please address these two points:

• In internal/cli/release.go, explicitly empty --tag bypasses conflict validation: forge release upload --tag='' v1.0.0 app.zip accepts the positional tag and proceeds to repository resolution. Check cmd.Flags().Changed("tag") and reject empty flag values so an unset script variable cannot silently select positional mode.
• In internal/cli/release_test.go, the new tests only call the parser helper and check that the flag exists. Add tests through command execution for both supported forms and invalid combinations, including an empty --tag, and assert the tag and file contents passed to UploadAsset. This will cover flag binding and the upload path together.