feat: add optional group field to server config

[ice616]

Closes #55.

Summary

Adds an optional, free-form group label to server definitions:

• .env: SSH_SERVER_<NAME>_GROUP=production
• TOML: group = "production"

The field round-trips through ConfigLoader.exportToToml() / exportToEnv(), and ssh_list_servers now returns it alongside name/host/user/defaultDir/description.

Why

As discussed in #55: today a server's group/environment membership only exists in the separate .server-groups.json file used by ssh_group_manage / ssh_execute_group for batch execution. That's a different concern (execution strategy — parallel/sequential/rolling, with servers referenced by name) from descriptive grouping.

Having group live on the server entry itself means:

• Exporting the config to another tool (or importing from one) carries the grouping information without needing a second file.
• Many external SSH clients / vault-style host managers already have a native group/folder concept per host — this gives a natural 1:1 field to map to/from when converting configs.
• ssh_list_servers can show group membership at a glance without cross-referencing .server-groups.json.

This is purely additive metadata — it does not touch .server-groups.json, ssh_group_manage, or ssh_execute_group, which keep working exactly as before. A follow-up could optionally let ssh_group_manage list derive a dynamic group from this field (similar to the existing all dynamic group), but that's out of scope here to keep this change minimal and low-risk.

Changes

• src/config-loader.js: read group from both the TOML and .env loaders; write it back in both exporters. Omitted entirely when unset, so existing configs and generated output are unaffected.
• src/index.js: ssh_list_servers includes group in its output (empty string when unset); description text updated to mention it.
• tests/test-config-field-names.js: extended EXPECTED_CAMEL_FIELDS and both the .env/TOML fixtures to cover group, keeping it locked in alongside the other camelCase fields this test already guards.
• README.md / CLAUDE.md: documented the new field in both the .env and TOML config references.
• CHANGELOG.md: added an [Unreleased] entry.

Testing

npm test

All suites pass except the pre-existing test:lifecycle failures (SIGTERM/SIGINT handling), which are unrelated to this change and reproduce identically on main without this branch (verified via git stash).

Also manually verified with a scratch TOML/.env fixture (load → export → reload round-trip) that group survives every direction: TOML→loader, .env→loader, loader→exportToToml, loader→exportToEnv, and export→reload.