Commit a change's stage so a shared plan agrees with itself #175

Why

specsync advertises workflow stages as "committed as metadata rather than
external labels" — the argument for keeping planning state in the repo instead
of in tracker labels. It is not true. Stage is written to
.specsync/metadata.json, and the whole .specsync/ directory is gitignored:
git ls-files finds zero tracked files under it.

Repo-local and solo, nobody notices. It breaks the moment planning is shared:
in a store, every teammate's clone has its own private idea of what stage each
change is in, and a stage set by one person is invisible to everyone else.

Surfaced while implementing openspec-store-support, which hit the same defect
for targets and resolved it by declaring them in a committed per-change
specsync.yml. Stage belongs in the same file.

What Changes

• Read stage and priority from the committed per-change specsync.yml.
• Keep reading .specsync/metadata.json as a fallback, and migrate on the next
  write, so existing local state is not lost.
• Keep the projection cache (refs.json, board.json, deps.json) gitignored:
  those are caches, and committing them would leak projection ids.

Impact

• Affected specs: change-state
• Affected code: store.go, change.go, the set-stage and set-priority commands