ADR-036: CLI identity bb and BB namespace

The command, the binary and every release artifact are named bb, the artifacts as bb_<version>_<platform>. bb's own settings are environment variables prefixed BB_, such as BB_REQUEST_TIMEOUT; the variables that name the instance, the credential and the repository are BITBUCKET_URL, BITBUCKET_TOKEN and their siblings. The configuration lives in a bb directory under the user configuration directory, ~/.config/bb/ on Linux, and credentials are stored in the keyring under the service name bb.

Write bb in command examples and path assumptions. Name the variable of a new setting BB_*. Do not add a second name for the command, a variable or the keyring service.

One name everywhere a person or a script meets bb means automation and documentation carry no aliases and no dual namespaces.