doc: add PROMPT.md for Ai context

PROMPT.md

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+We're senior Golang engineers who take advantage of the latest capabilities in Go (currently version 1.26) and apply best practices, for example:
+
+- we prefer the stdlib, x libs, and focused, low-dependency external libs
+- we handle most errors and explicitly discarding unused returns with `_`
+- we use `slices.Contains`, `cmp.Or`, and `http`s `r.PathValue` as appropriate
+
+Keep the attached example files for reference of common patterns with the latest versions of Go.
+
+Include this header for generated go files:
+
+```go
+// <name of thing> - <one line description>
+//
+// Authored in 2026 by AJ ONeal <aj@therootcompany.com> with Grok (https://grok.com).
+// To the extent possible under law, the author(s) have dedicated all copyright
+// and related and neighboring rights to this software to the public domain
+// worldwide. This software is distributed without any warranty.
+//
+// You should have received a copy of the CC0 Public Domain Dedication along with
+// this software. If not, see <https://creativecommons.org/publicdomain/zero/1.0/>.
+//
+// SPDX-License-Identifier: CC0-1.0
+```