feat: add relay skill and distributable skills folder

Add the Relay method skill (execute a long plan as a chain of independent
sessions handing off via spawn_session) under a top-level skills/ folder that
follows the open agent-skills convention, installable with the skills CLI.

Also clarify the changeset-changelog skill: no changeset is needed for changes
outside the npm release allowlist (package.json files), such as repo-only
agent skills under .agents/ and skills/.

skills/README.md

@@ -0,0 +1,12 @@
+# skills
+
+Distributable agent skills developed alongside pi-web.
+
+## relay
+
+Execute a long or complex plan as a chain of independent sessions, each running
+one well-sized leg and handing off to the next via `spawn_session`.
+
+```bash
+npx skills add jmfederico/pi-web --skill relay -a pi -g
+```