refactor(skills): remove ce-onboarding skill (#639)

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

tests/legacy-registry-invariants.test.ts

@@ -0,0 +1,32 @@
+import { describe, expect, test } from "bun:test"
+import fs from "fs/promises"
+import path from "path"
+import { STALE_SKILL_DIRS } from "../src/utils/legacy-cleanup"
+
+// If a previously-retired skill is re-added to the plugin, its entry must be
+// removed from STALE_SKILL_DIRS in the same PR. Leaving the entry in place
+// causes `cleanupStaleSkillDirs()` to fingerprint-match the just-installed
+// skill (the live plugin description becomes the fingerprint) and delete it
+// on every `bun install --to <target>` before the writer re-creates it.
+//
+// The sibling registry `EXTRA_LEGACY_ARTIFACTS_BY_PLUGIN["compound-engineering"].skills`
+// is NOT covered by this invariant: it can legitimately contain names of
+// current skills whose FLAT install path is stale (e.g. `ce-update`, which
+// now installs at `~/.codex/skills/compound-engineering/ce-update` but had a
+// pre-namespaced flat install at `~/.codex/skills/ce-update`). Sweeping the
+// flat path is correct even while the skill is current.
+
+const PLUGIN_ROOT = path.join(import.meta.dir, "..", "plugins", "compound-engineering")
+
+async function listCurrentSkillDirs(): Promise<Set<string>> {
+  const entries = await fs.readdir(path.join(PLUGIN_ROOT, "skills"), { withFileTypes: true })
+  return new Set(entries.filter((e) => e.isDirectory()).map((e) => e.name))
+}
+
+describe("legacy registry invariants", () => {
+  test("STALE_SKILL_DIRS contains no name matching a current plugin skill", async () => {
+    const current = await listCurrentSkillDirs()
+    const collisions = STALE_SKILL_DIRS.filter((name) => current.has(name))
+    expect(collisions).toEqual([])
+  })
+})