Add --output flag for json/table/raw modes

[joelhooks]

Problem

Currently output is JSON-only. Humans want pretty tables, scripts want raw values for piping.

Pattern from Go CLI Books

From "Building Modern CLI Applications in Go" (p.250-258):

type OutputFormatter interface {
    Format(data interface{}) (string, error)
}

type JSONFormatter struct{}
type TableFormatter struct{}
type RawFormatter struct{}

// Auto-detect based on TTY
if isatty.IsTerminal(os.Stdout.Fd()) {
    return TableFormatter{}
}
return JSONFormatter{}

Action Items

• Add --output flag to root command (json|table|raw)
• Create formatters in internal/output/
• Auto-detect TTY for default mode
• Update all commands to use formatter interface

Examples

secrets status --output json   # Structured JSON
secrets status --output table  # Pretty tables
secrets status --output raw    # Just values
secrets status                 # Auto: table if TTY, json otherwise

Book References

• Search: pdf-brain search "output format table json" --fts --expand 2000
• Search: pdf-brain search "isatty terminal detect" --fts --expand 2000

Priority

P1 - Better UX for both humans and agents