fix(ce-update): derive cache dir from CLAUDE_PLUGIN_ROOT parent (#645)

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/skills/ce-update.test.ts

@@ -0,0 +1,32 @@
+import { readFileSync } from "fs"
+import path from "path"
+import { describe, expect, test } from "bun:test"
+
+// Regression guard for https://github.com/EveryInc/compound-engineering-plugin/issues/556.
+//
+// `CLAUDE_PLUGIN_ROOT` is set by the Claude Code harness to the currently-loaded
+// plugin version directory (e.g. `~/.claude/plugins/cache/<marketplace>/compound-engineering/<version>`).
+// It is NOT the plugins cache root. Appending `/cache/<anything>/compound-engineering/`
+// onto it produces a path that never exists, which causes the cache-probe to fail
+// and emit the `__CE_UPDATE_CACHE_FAILED__` sentinel on every healthy install.
+//
+// This has regressed twice. Fail fast if the antipattern reappears.
+
+describe("ce-update SKILL.md", () => {
+  const skillPath = path.join(
+    process.cwd(),
+    "plugins/compound-engineering/skills/ce-update/SKILL.md",
+  )
+  const body = readFileSync(skillPath, "utf8")
+
+  test("does not append a /cache/<marketplace>/ suffix onto CLAUDE_PLUGIN_ROOT", () => {
+    // Matches any `${CLAUDE_PLUGIN_ROOT}/cache/<segment>/` or `${CLAUDE_PLUGIN_ROOT}/cache/<segment>"`
+    // pattern. The harness variable is already under `cache/<marketplace>/`,
+    // so concatenating another `cache/...` segment is always wrong.
+    const antiPattern = /\$\{CLAUDE_PLUGIN_ROOT\}\/cache\//
+    expect(
+      antiPattern.test(body),
+      "ce-update/SKILL.md reintroduced the ${CLAUDE_PLUGIN_ROOT}/cache/... antipattern — derive the cache dir from dirname \"${CLAUDE_PLUGIN_ROOT}\" instead.",
+    ).toBe(false)
+  })
+})