feat: rename bugfix-ship/requirement-flow to short-ship/long-ship with shared ship records

short-ship and long-ship share one ship-id record under .pouch/ship/<id>/.
specifier writes PRDs into spec/; delivery-loop appends evidence pointers.
Old names stay as deprecation stubs so existing installs keep working.

skills/short-ship/agents/openai.yaml

@@ -0,0 +1,6 @@
+interface:
+  display_name: "Short Ship"
+  short_description: "Ship a small change or bug fix through verification and delivery"
+  default_prompt: "Use $short-ship to implement this small change, verify it, and continue to the configured delivery stop."
+policy:
+  allow_implicit_invocation: true