Initial commit: nixbuild.net operator plugin

Haiku ops agent + 3 tools (nbshell expect+PTY admin-shell driver, nbapi
read-only HTTP client, nb-substituters guarded cache manager) + 2 skills
(nixbuild-settings, nixbuild-usage). Encodes the two-control-surface model
and the path-style-substituter-URL gotcha.

.gitignore

@@ -0,0 +1,3 @@
+.cache/
+.claude/
+*.log