refactor: improve CLI commands and authentication flow

This pull request refactors the CLI commands and improves the authentication flow for the PostgresAI tool.

• Renames the init command to prepare-db for clarity. This change better reflects the command's purpose of setting up the database for monitoring.
• Renames the quickstart command to local-install to provide a more accurate description of its functionality, which involves installing a local monitoring stack.
• Introduces auth --set-key option, allowing users to directly input their API key without using the browser-based OAuth flow.

postgres-ai|postgresai|pgai [global options] <command> [command options] [args]

Global options:
  --api-key <key>
  --api-base-url <url>
  --ui-base-url <url>

Commands:
  prepare-db [conn] [options]
    --db-url <url>                 (deprecated; use positional conn)
    -h, --host <host>
    -p, --port <port>
    -U, --username <username>
    -d, --dbname <dbname>
    --admin-password <password>
    --monitoring-user <name>
    --password <password>
    --skip-optional-permissions
    --verify
    --reset-password
    --print-sql
    --print-password

  mon <subcommand>
    local-install [options]
      --demo
      --api-key <key>
      --db-url <url>
      --tag <tag>
      -y, --yes
    start
    stop
    restart [service]
    status
    logs [service]
      -f, --follow
      --tail <lines>
    health
      --wait <seconds>
    config
    update-config
    update
    reset [service]
    clean
    shell <service>
    check
    targets <subcommand>
      list
      add [connStr] [name]
      remove <name>
      test <name>
    generate-grafana-password
    show-grafana-credentials

  auth [options]
    --set-key <key>                (сохранить API key без OAuth)
    --port <port>
    --debug

  add-key <apiKey>                 (deprecated; use "auth --set-key")
  show-key
  remove-key

  issues <subcommand>
    list
      --debug
      --json
    view <issueId>
      --debug
      --json
    post_comment <issueId> <content>
      --parent <uuid>
      --debug
      --json

  mcp <subcommand>
    start
      --debug
    install [client]               (cursor|claude-code|windsurf|codex)

  help

Closes #66 (closed)