P1.4: one-time YAML import in storage

When gosentry.json / jobs.json are absent, read a pre-migration
gosentry.yaml / jobs.yaml via private yaml-tagged shadow structs and
return the data unsaved; the existing SaveConfig/SaveJobs in OpenStore
then rewrite it as JSON. Replaces the placeholder that pointed configPath
at the legacy YAML file and tried to json.Unmarshal it.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/PRE-RELEASE-TASKS.md

@@ -83,7 +83,7 @@ These land together because both edit `domain/job.go` and `storage/store.go`.
 - [x] P1.1 — JSON struct tags
 - [x] P1.2 — `writeJSON` + JSON unmarshal
 - [x] P1.3 — `gosentry.json` / `jobs.json` paths; drop pysentry name
-- [ ] P1.4 — One-time YAML import
+- [x] P1.4 — One-time YAML import
 - [ ] P1.5 — Remove `SuccessExitCodes` across code
 - [ ] P1.6 — Update storage/runner/format tests + TESTS.md