docs(dev): delete developer/testing.md — contradicted current test doctrine

The Feb-era page described a wipe-all-test-users-per-run lifecycle,
which current doctrine explicitly forbids: the shared testuser is a
LIVE reader whose settings and history must survive every suite run,
cleanups are scoped preserve-lists, and hand-probes must be deleted.
Its recipes were also stale (no container runner, no pool rules, no
gates). The canonical home is docs/contributing/development.md
(architecture, setup, testing, project rules — kept fresh, 542 lines,
touched today), so the page was deleted rather than rewritten, with a
docRedirects entry sending old URLs there. No inbound doc links
existed.

internal/docs/http_handler.go

@@ -29,6 +29,12 @@ type HTTPHandler struct {
 var docRedirects = map[string]string{
 	"user/admin-guide":              "admin/admin-guide",
 	"user/admin-library-management": "admin/libraries",
+	// developer/testing.md (Feb-era) was deleted 2026-11-02: its
+	// wipe-test-users lifecycle contradicted current doctrine (live
+	// testuser is protected, cleanups are scoped) and its recipes were
+	// superseded by docs/contributing/development.md (architecture,
+	// setup, testing, project rules — kept fresh).
+	"developer/testing": "contributing/development",
 }
 
 // NewHTTPHandler creates a new docs HTTP handler