feat(cli): --json for auth login, logout and whoami

[darwin67]

Summary

This finishes the CLI half of RFD 3's machine-readable output item. #72 covered the paste commands; this adds --json to the three auth commands. Every CLI command now takes --json and prints one line of JSON on stdout.

Command	JSON
auth whoami --json	{server, user: {id, email}, token: {id, name, inserted_at}}
auth login --json	{status, profile, server, user, token}. status is logged_in, already_logged_in or environment (authenticated through TEXTBIN_TOKEN).
auth logout --json	{profile, server, revoked, logged_out}. profile is null when only TEXTBIN_TOKEN was revoked. server is set only when a token was revoked.

• The API token secret is never printed. token is the metadata the server returns: id, name and creation time. A test checks the secret isn't in the output.
• The email prompt now goes to stderr, like the password prompt, so stdout carries only the result. Human output is otherwise unchanged.

Commits

1. refactor: moves print_json to the crate root so the auth commands can use it, and makes the client's identity types serializable.
2. feat: adds the three --json flags, with tests.

Test plan

• cargo fmt --check and cargo clippy --workspace --all-features -- -D warnings pass.
• cargo test --workspace: 63 tests, 0 failures. New tests:
  • The login JSON shape, including that the secret isn't present.
  • Argument parsing for --json on all three auth commands.
• The refactor commit builds and passes on its own.

🤖 Generated with Claude Code

https://claude.ai/code/session_019fnjMN7WpEjQJD2zveakV6

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Generated by Claude Code

[darwin67]

Oracle review complete: signed off, no blockers. Reviewed auth JSON output, secret-free metadata serialization, stderr/terminal prompting, and propagation of authentication/storage/revocation errors before success output. No code correction was required. Validation: cargo fmt --check, cargo clippy --workspace --all-features -- -D warnings, and cargo test --workspace passed (63 tests). Review session: https://ampcode.com/threads/T-01a12594-e2f1-75be-b343-9239564f5c84

CI is green at the reviewed head. For PR78 this includes the fresh post-fix memory/local/S3 test matrix, Rust tests, formatting/linting, and production-container build; release/publication jobs skipped as expected.