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/STANDARDS.md

@@ -29,12 +29,24 @@ change to their shape has to stay compatible on its own.
   one helper that every consumer shares (`JobListView.IsCompact`, `ui.themeFor`),
   and is normalized before being written back, so the file never gains a value
   no reader understands.
+- A renamed key keeps the old field on `Config` (tagged `omitempty`) purely so
+  it can still be read. `storage.loadOrCreateConfig` converts it to the new
+  field and clears it, so the retired key disappears on the next save. See
+  `Config.JobsDir` → `Config.JobsFile`. Where the new field has a non-empty
+  default, clear that default before unmarshalling, or "the file omits it" and
+  "the file sets it" become indistinguishable and the conversion never runs.
 - Each of the three gets a test: the default in `storage`, the normalization in
   `domain`, and a round-trip through the real config file in `app`.
 
 ## Intentional behavior (not bugs)
 
 - `RunNow` is allowed during global pause and for disabled jobs.
+- Selecting a jobs file that already exists **loads** it: its jobs replace the
+  in-memory list, which is the only way the user can switch between job lists. A
+  path with no file behind it receives the current jobs (rename/relocate). The
+  switch is refused while a job is running, because adoption drops every runtime
+  and a finishing run would then write its result onto whichever job inherited
+  its ID.
 - Sequential mode runs jobs FIFO by order in `jobs.json`.
 - Scheduler tick is 1s — sub-second `@every` intervals are not supported.
 - Command timeout defaults to no timeout globally (`Config.DefaultTimeoutSeconds`