P3.1: Add ExecutionMode/OverlapPolicy to Config; add Pending to JobRuntime

Introduces ExecutionMode (parallel/sequential) and OverlapPolicy
(skip/queue) types and constants in domain/config.go, wires defaults
(parallel/skip) into loadOrCreateConfig and the normalization pass, and
adds validation in validateConfig. Adds Pending bool to JobRuntime as
the flag P3.3 will use to re-run a queued overlap. Marks P3.1 done.

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

docs/PRE-RELEASE-TASKS.md

@@ -94,7 +94,7 @@ These land together because both edit `domain/job.go` and `storage/store.go`.
 - [x] P2.4 — `.gitignore` / `.dockerignore`
 
 ### Phase 3 — Task-queue model + settings
-- [ ] P3.1 — Config/runtime fields + defaults
+- [x] P3.1 — Config/runtime fields + defaults
 - [ ] P3.2 — Split dispatch into `app/run.go`
 - [ ] P3.3 — Rework `RunDue`/`executeRun` for mode + overlap policy
 - [ ] P3.4 — Settings Queue selects