feat(scheduler): atomic JSON job store

jobs.json is { "version": 1, "jobs": [...] } in VolcanicJson camelCase,
written to jobs.json.<pid>.tmp and moved into place through one semaphore.
A corrupt file is renamed jobs.json.corrupt-<timestamp> and loading starts
empty. Each entry is deserialised on its own, so an entry with an unknown
kind or a missing required field is dropped with a warning like any other
invalid job instead of setting the whole file aside.

tests/Volcanic.AgenticCore.Extensions.Tests/Scheduler/JobStoreTests.cs

@@ -0,0 +1,114 @@
+using Microsoft.Extensions.Logging.Abstractions;
+using Microsoft.Extensions.Time.Testing;
+using Volcanic.AgenticCore.Extensions.Scheduler;
+
+namespace Volcanic.AgenticCore.Extensions.Tests.Scheduler;
+
+public sealed class JobStoreTests : IDisposable
+{
+    private static readonly DateTimeOffset Now = new(2026, 9, 26, 14, 3, 0, TimeSpan.Zero);
+    private readonly string _root = Directory.CreateTempSubdirectory("jobstore-").FullName;
+    private readonly FakeTimeProvider _time = new(Now);
+
+    private string StorePath => Path.Combine(_root, "scheduler", "jobs.json");
+    private JobStore Store() => new(StorePath, _time, NullLogger.Instance);
+
+    internal static ScheduledJob Reminder(string id, string target = "111") => new()
+    {
+        Id = id, Target = target, Name = "Call the dentist", Kind = JobKind.Reminder, Text = "Call the dentist",
+        Schedule = new JobSchedule(Now.AddDays(1), null), CreatedAt = Now,
+    };
+
+    public void Dispose()
+    {
+        try { Directory.Delete(_root, recursive: true); } catch (IOException) { }
+    }
+
+    [Fact]
+    public async Task Missing_file_loads_empty() => Assert.Empty(await Store().LoadAsync(CancellationToken.None));
+
+    [Fact]
+    public async Task Round_trip_keeps_jobs_and_leaves_no_temp_file()
+    {
+        var watcher = new ScheduledJob
+        {
+            Id = "w8r4t", Target = "111", Name = "Price check", Kind = JobKind.Watcher, Command = "scripts/price.sh",
+            TimeoutSeconds = 60, Schedule = new JobSchedule(null, "*/10 * * * *"), CreatedAt = Now,
+            LastRunAt = Now, LastResult = new JobResult(false, 1, false, "boom", Now),
+        };
+        await Store().SaveAsync([Reminder("k7m2q"), watcher], CancellationToken.None);
+
+        Assert.Equal([Reminder("k7m2q"), watcher], await Store().LoadAsync(CancellationToken.None));
+        Assert.Equal(["jobs.json"], Directory.GetFiles(Path.GetDirectoryName(StorePath)!).Select(Path.GetFileName));
+    }
+
+    [Fact]
+    public async Task File_format_is_versioned_camel_case()
+    {
+        await Store().SaveAsync([Reminder("k7m2q")], CancellationToken.None);
+        var json = await File.ReadAllTextAsync(StorePath);
+        Assert.Contains("\"version\": 1", json);
+        Assert.Contains("\"jobs\": [", json);
+        Assert.Contains("\"kind\": \"reminder\"", json);
+        Assert.NotEqual(0xEF, (await File.ReadAllBytesAsync(StorePath))[0]); // no BOM
+    }
+
+    [Fact]
+    public async Task Corrupt_file_is_set_aside_and_loading_starts_empty()
+    {
+        Directory.CreateDirectory(Path.GetDirectoryName(StorePath)!);
+        await File.WriteAllTextAsync(StorePath, "{ not json");
+
+        Assert.Empty(await Store().LoadAsync(CancellationToken.None));
+        Assert.False(File.Exists(StorePath));
+        Assert.Equal("{ not json", await File.ReadAllTextAsync(StorePath + ".corrupt-20260926140300"));
+    }
+
+    [Fact]
+    public async Task Invalid_entries_are_dropped()
+    {
+        Directory.CreateDirectory(Path.GetDirectoryName(StorePath)!);
+        await File.WriteAllTextAsync(StorePath, """
+            { "version": 1, "jobs": [
+              { "id": "good1", "target": "111", "name": "ok", "kind": "reminder", "text": "hi", "schedule": { "cron": "0 9 * * *" }, "createdAt": "2026-09-26T14:03:00+00:00" },
+              { "id": "both1", "target": "111", "name": "both", "kind": "reminder", "text": "hi", "schedule": { "at": "2026-09-27T09:00:00+00:00", "cron": "0 9 * * *" }, "createdAt": "2026-09-26T14:03:00+00:00" },
+              { "id": "nocmd", "target": "111", "name": "watch", "kind": "watcher", "schedule": { "cron": "* * * * *" }, "createdAt": "2026-09-26T14:03:00+00:00" },
+              { "id": "good1", "target": "111", "name": "duplicate id", "kind": "reminder", "text": "hi", "schedule": { "cron": "0 9 * * *" }, "createdAt": "2026-09-26T14:03:00+00:00" }
+            ] }
+            """);
+
+        var jobs = await Store().LoadAsync(CancellationToken.None);
+
+        Assert.Equal("ok", Assert.Single(jobs).Name);
+    }
+
+    [Fact]
+    public async Task An_entry_that_does_not_deserialise_is_dropped_without_setting_the_file_aside()
+    {
+        Directory.CreateDirectory(Path.GetDirectoryName(StorePath)!);
+        await File.WriteAllTextAsync(StorePath, """
+            { "version": 1, "jobs": [
+              { "id": "good1", "target": "111", "name": "ok", "kind": "reminder", "text": "hi", "schedule": { "cron": "0 9 * * *" }, "createdAt": "2026-09-26T14:03:00+00:00" },
+              { "id": "kind1", "target": "111", "name": "bad kind", "kind": "alarm", "text": "hi", "schedule": { "cron": "0 9 * * *" }, "createdAt": "2026-09-26T14:03:00+00:00" },
+              { "id": "noat1", "target": "111", "name": "no createdAt", "kind": "reminder", "text": "hi", "schedule": { "cron": "0 9 * * *" } },
+              null,
+              42
+            ] }
+            """);
+
+        var jobs = await Store().LoadAsync(CancellationToken.None);
+
+        Assert.Equal("good1", Assert.Single(jobs).Id);
+        Assert.True(File.Exists(StorePath));
+    }
+
+    [Fact]
+    public async Task Concurrent_saves_are_serialised()
+    {
+        var store = Store();
+        await Task.WhenAll(Enumerable.Range(0, 20).Select(i => Task.Run(() => store.SaveAsync([Reminder($"id{i:000}")], CancellationToken.None))));
+
+        Assert.Single(await Store().LoadAsync(CancellationToken.None));
+        Assert.Empty(Directory.GetFiles(Path.GetDirectoryName(StorePath)!, "*.tmp"));
+    }
+}