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/CHANGELOG.md

@@ -2,6 +2,48 @@
 
 All notable GoSentry changes are recorded in this file.
 
+## 0.15.0 - 2026-07-26
+
+**Settings points at the jobs file itself, not the folder holding it.**
+
+**Settings:**
+
+- The **Jobs directory** row is now a **Jobs file** row. Browse opens a file
+  picker filtered to `.json` instead of a folder picker, so the job list can
+  live under any file name — `team-jobs.json`, one file per machine, a file
+  shared over a network drive — rather than a fixed `jobs.json` per folder. The
+  field stays editable, which is how a file that does not exist yet is named.
+- **Selecting an existing jobs file now loads it.** Previously the current job
+  list was written over whatever was at the new path, which made it impossible
+  to switch to an existing jobs file — its contents were destroyed on Save. Now
+  an existing file wins: its jobs are loaded, normalized, and replace the loaded
+  list, with runtimes, parsed schedules, next-run times, and log-seeded
+  statistics rebuilt around them. A path with no file behind it still receives
+  the current jobs (and its folder is created), which is how the jobs file is
+  renamed or relocated. History records `Jobs loaded — N jobs from <path>`,
+  since the switch happens without a prompt.
+- Switching to a different jobs file is refused while a job is running: adoption
+  discards every runtime, and a run finishing afterwards would write its result
+  onto whichever job inherited its ID. Settings unrelated to the jobs file still
+  save normally during a run.
+- Saving a path with no file name (a trailing separator, `.`, `..`) is rejected
+  with "jobs file must include a file name" instead of failing later with an
+  opaque OS error.
+
+**Configuration:**
+
+- `Config.JobsDir` / `jobs_dir` is replaced by `Config.JobsFile` / `jobs_file`,
+  which holds the full path including the file name; the default is
+  `"jobs.json"`, resolved against the program folder as before. `Paths.JobsDir`
+  is now derived from the configured file so job saves still create the folder.
+- A `gosentry.json` written by an earlier version is migrated on load: its
+  `jobs_dir` is joined with `jobs.json`, which is the exact file that version
+  used, and the retired key is dropped when the config is rewritten.
+- New `app.JobsLoaded{Path, Count}` event, emitted when a selected jobs file
+  replaces the job list; the UI turns it into the History entry. New
+  `storage.LoadJobsFile`, which reads and normalizes a jobs file and reports a
+  missing one as "not found" instead of seeding it the way startup does.
+
 ## 0.14.0 - 2026-07-26
 
 **Compact job list view, "no timeout" at both timeout levels, and an Open