feat(auth): add JSON output to auth status

Description

Adds --output json (-F json) and --jq to glab auth status, using the existing cmdutils.EnableJSONOutput helper. Tools that drive glab (editor extensions, desktop apps, scripts) can now find which GitLab instances the user is signed in to, and whether those credentials still work, without parsing the human-readable text on stderr.

$ glab auth status --all --output json
{"hosts":[{"host":"gitlab.com","state":"success","user":"jdoe","token_source":"keyring","git_protocol":"ssh","api_protocol":"https","api_endpoint":"https://gitlab.com/api/v4/","graphql_endpoint":"https://gitlab.com/api/graphql/"}]}

Each checked instance reports host, state (success or error), user, error, token_source (keyring, environment, config_file, or none when no token is stored), git_protocol, api_protocol, api_endpoint, graphql_endpoint, subfolder, ssh_host, and warnings, as proposed in the issue. warnings lists hints that text mode prints, such as an OAuth host with no refresh token, and is omitted when empty.

Behavior, following the proposal:

• Host selection is unchanged: --hostname, --all, and the current context work the same as in text mode.
• The JSON never contains the token. --show-token together with --output json is rejected.
• The exit code is unchanged: it is 1 when any instance fails to authenticate. In JSON mode, stdout holds a single JSON document (the failure is signaled with cmdutils.SilentError, so the error handler does not append a second JSON object).
• Text output is unchanged. In JSON mode, the text hints on stderr are not printed; the incomplete-OAuth hint is reported in warnings instead.
• tokenSourceKind is the single token-source classifier, and the text output (tokenStorageDescription, isPlaintextTokenSource) is built on it, so text and JSON cannot drift apart.

Related Issues

Closes #8576 (closed)

How has this been tested?

• New unit tests in internal/commands/auth/status/status_test.go:
  • mixed success and failure across two hosts: asserts each hostStatus field after json.Unmarshal, the SilentError return (exit 1), empty stderr, and no token in stdout
  • single host with a token from GITLAB_TOKEN: asserts token_source is environment and the command succeeds
  • --show-token --output json is rejected
  • error branches in JSON mode: keyring read failure, API client setup failure, and token_source: "none" when no token is stored
  • warnings is set for an OAuth host with no oauth2_refresh_token
  • table test for tokenSourceKind covering every branch, including keyring
• Existing text-mode tests pass unchanged.
• go test ./internal/commands/auth/... ./internal/commands/ ./cmd/...
• golangci-lint run ./internal/commands/auth/...
• go run ./scripts/comment-overlap
• make gen-docs (regenerated docs/source/auth/status.md is included in the commit)

To try it locally:

make build
./bin/glab auth status --all --output json
./bin/glab auth status --output json --jq '.hosts[] | select(.state == "error") | .host'
./bin/glab auth status --show-token --output json   # rejected

Screenshots (if appropriate):

Not applicable.