feat(completion): complete flag values and stop offering file names

Every value completed to file names, because no command registered a
completion. Values without a completer now complete nothing, and only
path flags (--*-file, --input, --attachment, --asset, --file, --ssh-key,
--dir, --cwd) and arguments named file or directory complete paths.

--json completes the fields of the command, also after a comma. --host,
--user, --remote and -R complete from hosts.yml, Git remotes and the
instance. enumFlag and enumValues give a flag one list of values that
validation (case-insensitive) and completion share, and instance
completers for issues, pull requests, labels, milestones, users,
branches, tags, releases, organizations, teams, workflows, runs, secrets
and variables are ready for commands to attach. They give up after two
seconds, never prompt, retry or refresh an OAuth token, and drop values
with control characters.

tea completion bash now says to use eval "$(tea completion bash)",
because bash 3.2 on macOS cannot source <(...), and names the
bash-completion package; the zsh help says to run compinit first. The
README matches.

Signed-off-by: bircni <bircni@icloud.com>

README.md

@@ -69,13 +69,14 @@ Alternatively, `tea auth login --host https://your-instance --oauth` authorizes
 
 ### Shell completion
 
-If you installed from source or the package does not provide the completions with it you can add them yourself with `tea completion <shell>` command. To generate the completions run one of the following commands depending on your shell.
+If you installed from source or the package does not provide the completions with it you can add them yourself with `tea completion <shell>` command. Completion offers commands, flags, flag values such as `--state` and `--json` fields, and values from your instance such as repositories. Add the line for your shell to its startup file:
 
 ```shell
-# .bashrc
-source <(tea completion bash)
+# ~/.bashrc (on macOS, ~/.bash_profile)
+eval "$(tea completion bash)"
 
-# .zshrc
+# ~/.zshrc, after compinit
+autoload -U compinit && compinit
 source <(tea completion zsh)
 
 # fish
@@ -85,6 +86,8 @@ tea completion fish > ~/.config/fish/completions/tea.fish
 tea completion powershell | Out-String | Invoke-Expression
 ```
 
+The bash script needs the bash-completion package. On macOS, install `bash-completion` with Homebrew for the system bash 3.2 (or `bash-completion@2` for a Homebrew bash 4.2+), and use `eval` as above: bash 3.2 cannot `source <(...)`. Without `compinit`, zsh reports `command not found: compdef`. `tea completion <shell> --help` shows these steps and how to install the script system-wide.
+
 ### Man Page
 
 The hidden command `tea man` can be used to generate the `tea` man page.