Flatten the bundled Matrix messaging skill layout

.Rbuildignore

@@ -1,6 +1,7 @@
 ^\.git$
 ^\.github$
 ^\.claude$
+^\.agents$
 ^CLAUDE\.md$
 ^cran-comments\.md$
 ^README\.Rmd$

NEWS.md

@@ -1,5 +1,12 @@
 # mx.client 0.2.0.8
 
+## Documentation
+
+* Move the bundled skill to `inst/skills/matrix-messaging/`, installed at
+  `system.file("skills", "matrix-messaging", package = "mx.client")`.
+  The instruction body is unchanged; direct links to the old nested
+  `skills/mx.client/matrix-messaging` path need updating.
+
 ## New
 
 * Missing Megolm sessions now create persistent `m.room_key_request`

inst/tinytest/test_skills.R

@@ -0,0 +1,14 @@
+library(tinytest)
+
+root <- system.file("skills", package = "mx.client")
+expect_true(nzchar(root))
+skill <- file.path(root, "matrix-messaging", "SKILL.md")
+expect_true(file.exists(skill))
+expect_false(file.exists(file.path(root, "mx.client",
+                                  "matrix-messaging", "SKILL.md")))
+entries <- list.files(root, pattern = "^SKILL[.]md$", recursive = TRUE)
+expect_equal(entries, "matrix-messaging/SKILL.md")
+body <- readLines(skill, warn = FALSE)
+expect_equal(body[[1L]], "---")
+expect_true("name: matrix-messaging" %in% body)
+expect_true(any(grepl("^description:", body)))