refactor: split jobs_view, drop YAML migration, update docs (T6.1-T6.4)

T6.1: split jobs_view.go into three files — jobs_view_helpers.go (pure
helpers) and jobs_view_details.go (detailsPanel struct with widget
creation, update, clear, and container methods) — bringing jobs_view.go
from 459 to ~200 lines.

T6.2: remove stale YAML upgrade note from README; drop *.yaml from
.dockerignore.

T6.3: delete YAML shadow structs (yamlConfig/yamlJob/yamlJobsFile),
importYAMLConfig/importYAMLJobs, legacy path constants, and all
YAML-import tests; run go mod tidy to remove go.yaml.in/yaml/v4.

T6.4: refresh ARCHITECTURE.md — JSON storage references, new Key Domain
Concepts section (per-job overlap policy, run-time statistics + log
seeding, persisted pause flag, jobs_view split).

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

README.md

@@ -75,10 +75,6 @@ include the run timestamp and job name:
 20260614-224306_Hello_scheduler.log
 ```
 
-**Upgrading from an earlier build:** if `gosentry.yaml` / `jobs.yaml` exist
-next to the executable, GoSentry imports them once and rewrites the data as
-JSON. The old `.yaml` files are left untouched.
-
 ## Schedules
 
 Interval schedules using Go duration syntax: