`adr list` alternative output formats

[corani]

Instead of only a table format it might be nice to also support e.g. JSON format

[corani]

Implemented in PR (branched from master). The --format / -f flag is now available on adr list, adr find, and adr show:

• md (default) — rendered markdown via glamour, same as before
• raw — unrendered markdown (frontmatter as a table + body), suitable for piping to other tools
• json — structured JSON with frontmatter fields (number, title, status, date) as separate keys; adr show also includes body (frontmatter stripped, trimmed)

Invalid format values are rejected with an error message.