fix: wallet list routes human-readable summary to stderr, keeps only JSON on stdout

[Nicolai1205]

Fixes #154

Rebased version of #289 (which had a merge conflict) onto current main.

What

nansen wallet list was writing human-readable text to stdout alongside JSON, breaking agent pipelines that use | jq ..

How

• Route human-readable wallet summary to stderr via ttyOutput = console.error dep
• Keep clean JSON output on stdout only
• Interactive users still see the summary (it goes to stderr which terminals display)
• Agent pipelines work: nansen wallet list | jq .

Test

nansen wallet list        # human-readable summary on stderr, JSON on stdout
nansen wallet list | jq . # parses cleanly

All 929 tests pass.

[erhnysr]

Reopening this line of work in #576. This PR was closed on the understanding that the fix had already landed via #397 — but #397's diff only extracted a requireWalletFile() helper for the existence checks in showWallet/exportWallet/setDefaultWallet/deleteWallet; it never touched listWallets or the list handler. A full git log -L :listWallets:src/wallet.js trace confirms no commit ever introduced stderr/stdout separation for it, so the bug is still live on main. #576 is a fresh fix (summary → stderr, structured { wallets } → clean JSON on stdout) with a failing-first regression test and the full suite green.