Say a new Gadget starts empty, and name missing files in editFile (#600)

In 24 of 40 eval trials, the agent read server.js and client.js right
after creating an empty Gadget, because the system prompt said every
Gadget has both files. The prompt now says to create them, and that a
new Gadget has none unless it came from a blueprint.

editFile checked that the agent had read a file before checking that
the file exists, so a mistyped name got "You must read a file before
you can edit it." It now names the missing file. The check reuses
readFile's lookup, moved into readToolFile, and runs only when the edit
is refused. The editFile description now states the read rule.