feat: select the jobs file itself in Settings

The Jobs directory row named a folder and assumed the file inside it was
called jobs.json. It is now a Jobs file row: Browse opens a file picker
filtered to .json, the field stays editable so a file that does not exist yet
can be typed, and the job list can live under any name.

Config.JobsDir/jobs_dir becomes Config.JobsFile/jobs_file, holding the whole
path; Paths.JobsDir is derived from it so saves still create the folder. An
older gosentry.json is migrated on load by joining its jobs_dir with
jobs.json — the exact file that version used — and the retired key is dropped
when the config is rewritten. The default clears before unmarshalling, or a
file that omits jobs_file and a file that sets it would be indistinguishable
and the migration would never run.

Saving used to write the current job list over whatever was at the new path,
which made switching to an existing jobs file impossible: its contents were
destroyed. An existing file now wins. Its jobs are loaded, normalized, and
adopted, with runtimes, schedule cache, next-run times and log-seeded
statistics rebuilt around them by adoptJobsLocked — the same helper NewService
now uses, so construction and adoption cannot drift. A path with no file
behind it still receives the current jobs, which is how the file is renamed or
relocated. The new file is read before anything is written, so an unparsable
one leaves both the config and the jobs untouched.

Adoption drops every runtime, and a run finishing afterwards would write its
result onto whichever job inherited its ID, so the switch is refused while a
job is running. Unrelated settings still save during a run. Because the
replacement happens without a prompt, the Service emits JobsLoaded with the
path and count, and History carries the receipt.

A path that names only a folder (trailing separator, a dot, or two dots) is
rejected with a validation error instead of failing later with an opaque OS
error.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/TESTS.md

@@ -130,6 +130,11 @@ Tests all mutating operations on the Service, scheduler integration, and setting
 |------|---------|
 | `TestUpdateSettingsPersistsAndValidates` | Verifies that `UpdateSettings` persists a valid config and rewrites autostart if needed. |
 | `TestUpdateSettingsRejectsInvalidConfigs` | Verifies that `UpdateSettings` returns validation errors without persisting. |
+| `TestHasFileName` | Verifies the jobs-file path check: a file name passes; a trailing separator, `.`, and `..` do not. |
+| `TestUpdateSettingsWritesJobsToTheNewFile` | Verifies that changing `JobsFile` re-resolves `Paths.JobsPath` and writes the loaded jobs to the new file, creating its folder. |
+| `TestUpdateSettingsAdoptsExistingJobsFile` | Verifies that selecting a jobs file that already exists replaces the job list with its contents, rebuilds runtimes, and emits `JobsLoaded`. |
+| `TestUpdateSettingsKeepsJobsWhenTheNewFileIsMissing` | Verifies that a path with no file behind it receives the current jobs instead (the rename/relocate case). |
+| `TestUpdateSettingsRefusesJobsFileSwitchWhileRunning` | Verifies that switching the jobs file is refused (and not persisted) while a job runs, while unrelated settings still save. |
 | `TestPrependLogCapsActivityList` | Verifies that the activity log never grows beyond its maximum cap. |
 
 ---
@@ -204,6 +209,9 @@ Tests JSON round-tripping and default generation.
 | `TestConfigRoundTrip` | Verifies that settings saved to JSON are reloaded with identical field values. |
 | `TestNormalizeJobsFillsDefaults` | Verifies that `normalizeJobs` assigns sequential IDs and sets default name, schedule, and command for jobs missing those fields. |
 | `TestLoadOrCreateConfigCreatesDefaultsOnFirstRun` | Verifies that a missing config file is created with sane defaults and a sample job. |
+| `TestLoadOrCreateConfigMigratesJobsDir` | Verifies that a pre-0.15 `jobs_dir` becomes `jobs_file` pointing at the same `jobs.json`, and that the retired key is not written back. |
+| `TestLoadJobsFileReportsMissingWithoutCreating` | Verifies that `LoadJobsFile` reports a missing file as not-found without creating or seeding it, and normalizes the jobs it does load. |
+| `TestApplyConfigPathsDerivesJobsDir` | Verifies that the configured jobs file resolves against the program folder and that `Paths.JobsDir` is derived from it. |
 | `TestJobsJSONDoesNotPersistRuntimeNoise` | Verifies that `jobs.json` does not persist runtime state (LastRun, NextRun, etc.). Only durable job fields are stored. |
 
 ---