Implement output formatters (table, JSON, CSV) with field selection

[ran-codes]

Description

Build the output formatting layer that all commands will use. Supports table, JSON, and CSV output with TTY auto-detection and field selection.

Requirements

• NFR-1.1: Every command supports --output json|table|csv
• NFR-1.2: Default to table when stdout is TTY, json when piped
• NFR-1.3: --fields flag to select specific columns
• NFR-1.4: Errors output as structured JSON to stderr when --output json
• Design Decision D-2: Output format auto-detection

Acceptance Criteria

• internal/output/formatter.go — Formatter interface with Format(data interface{}, format string, fields []string) (string, error)
• internal/output/table.go — table formatter using olekukonez/tablewriter
• internal/output/json.go — JSON formatter (pretty-print for TTY, compact for pipe)
• internal/output/csv.go — CSV formatter with header row
• TTY auto-detection using mattn/go-isatty
• --fields parsing and column filtering
• Unit tests for each formatter with sample data

Dependencies

• Requires: github.com/olekukonez/tablewriter, github.com/mattn/go-isatty
• Blocked by: Define model structs for records, communities, licenses, and stats #7 (model structs)