fix(config): drop the willingness checkbox from the missing-plugin form

[KrimsN]

Summary

• Remove the "willing to write and test this plugin myself" checkbox. The dnspatch issue forms have no such field, and GitHub cannot prefill a checkbox from the URL, so the answer was appended to "Why this one" instead.
• The reason is now passed to the form as typed.

Linear: Part of DNS-59

Test plan

• tsc, biome, unit tests