#2717 gatorwalk-factory: rename plugin/eject to template/apply (GW-20)

Part of the gatorwalk-factory rebuild of @swamp/software-factory (gatorwalk-factory/ in swamp-extensions). Read gatorwalk-factory/README.md and DESIGN.md first. Follows GW-12 (#2663, stage plugins by eject); blocks GW-13 (#2684, starter plugins) so the starter set is authored with the final names.

GW-12 was narrowed to eject only: a lifecycle copies a plugin's stages and owns them, and never keeps a reference. The names still describe the dropped design. "Eject" comes from create-react-app / Angular CLI / Expo, where you leave a managed dependency by copying its config out; here there is no dependency to leave, and from the lifecycle's side we are using a template. "Plugin" likewise suggests a live dependency.

Rename, throughout gatorwalk-factory/:

• plugin -> template (stage template in prose): PluginSchema, the plugin holder model type (@swamp/gatorwalk-factory/plugin -> @swamp/gatorwalk-factory/template), _lib/plugin_instance.ts, instantiatePlugin, extensions/models/plugin.ts and its tests, testdata/plugins/, GW-9 findings and error text that say plugin (e.g. exit-unreachable wording).
• eject -> apply (a lifecycle uses a template; the holder method that copies it in is apply): the eject method, ejectMethod in _lib/work_item_ops.ts, _lib/eject.ts, the ejected-lifecycle record (-> applied-lifecycle), log/error messages.
• DESIGN.md ("Stage plugins: eject only" and every other mention), README.md, the gatorwalk-factory skill and its references, integration/ tests.

Pure rename: no behaviour change. Nothing is published, so no compatibility shims or aliases for the old type, method or record names.

Tests: the existing unit, GW-9 and integration/ suites pass unchanged apart from names; a grep for eject and plugin in gatorwalk-factory/ finds nothing left over (other than a sentence in DESIGN.md recording the rename and why).