P1.3: rename storage files to gosentry.json / jobs.json

- ConfigFileName → "gosentry.json", JobsFileName → "jobs.json"
- Remove exported LegacyConfigFileName ("pysentry.yaml")
- Add unexported legacyYAMLConfigFileName / legacyYAMLJobsFileName for
  the upcoming one-time YAML import (P1.4)
- Update store.go fallback path and comments to describe YAML→JSON
  migration rather than the old PySentry→GoSentry rename
- Align store_test.go references and test comments

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/PRE-RELEASE-TASKS.md

@@ -82,7 +82,7 @@ These land together because both edit `domain/job.go` and `storage/store.go`.
 ### Phase 1 — Storage JSON + exit-code removal
 - [x] P1.1 — JSON struct tags
 - [x] P1.2 — `writeJSON` + JSON unmarshal
-- [ ] P1.3 — `gosentry.json` / `jobs.json` paths; drop pysentry name
+- [x] P1.3 — `gosentry.json` / `jobs.json` paths; drop pysentry name
 - [ ] P1.4 — One-time YAML import
 - [ ] P1.5 — Remove `SuccessExitCodes` across code
 - [ ] P1.6 — Update storage/runner/format tests + TESTS.md