Fleet Refactor: One :argdo, Zero Collateral

A config key has been renamed, and every file in the fleet needs the change. /root/case/assignment.env names the fleet directory, the old key and the new key (they differ per session, as do the files).

In every fleet file, /root/fleet/*.conf, each active line that sets the old key (OLDKEY=<value> at the start of a line) must become NEWKEY=<value>, with the same value. Nothing else may change:

• other keys that merely contain the old name are different settings;
• commented-out lines (# OLDKEY=…) are history, so leave them as written;
• /root/fleet/archive/ and /root/fleet/template.conf.example are not part of the fleet;
• some fleet files don't set the key at all, and that's fine.

In vim: load the fleet as the argument list, then one :argdo with an anchored substitution and | update. Grading compares every file with the exact expected content, so any tool passes, but only a precise edit does.