fix(plan): use .specify/feature.json to allow /speckit.plan on custom git branches (#2305) (#2349)

* fix: allow plan setup to use feature metadata on custom branches

* fix: harden feature metadata validation

* fix: use portable feature metadata path

Made-with: Cursor

* fix: share feature.json parser and make path compare OS aware

* test: isolate setup plan subprocess environment

* fix: normalize feature metadata paths with pwd -P