CLI: implement `markdown` output for the list commands

[xarmian]

Background

Follow-up to #851 (option (a)). #851 validated the --format enum and made the help text honest about where markdown is actually supported (shipped in #893). This issue covers the other half: actually implementing markdown output everywhere the flag advertises it, so the format becomes honestly global rather than honestly-partial.

Today markdown is implemented on a handful of commands (e.g. item show, project changelog) but not on the list commands. The flag help currently reads: table, json (markdown on select commands, e.g. item show, project changelog).

Fix

Implement markdown rendering for the list commands that lack it:

• pad item list
• pad collection list
• (and any other list-style command missing markdown)

Once markdown is supported on the list commands, update the --format flag help in cmd/pad/main.go to advertise markdown as a first-class global format again.

Pointers

• cmd/pad/main.go — the persistent --format flag + output helpers.
• Look at how item show / project changelog implement their markdown branch for the pattern to follow.

/cc @b4rk13 — this is the option (a) follow-up you proposed on #851.