P3.1: Add ExecutionMode/OverlapPolicy to Config; add Pending to JobRuntime

Introduces ExecutionMode (parallel/sequential) and OverlapPolicy
(skip/queue) types and constants in domain/config.go, wires defaults
(parallel/skip) into loadOrCreateConfig and the normalization pass, and
adds validation in validateConfig. Adds Pending bool to JobRuntime as
the flag P3.3 will use to re-run a queued overlap. Marks P3.1 done.

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

src/domain/config.go

@@ -5,17 +5,41 @@ package domain
 // launches omit this flag and open the normal window.
 const StartInTrayArgument = "--start-in-tray"
 
+// ExecutionMode controls whether due jobs run concurrently or one at a time.
+type ExecutionMode string
+
+const (
+	// ExecutionModeParallel allows all due jobs to start simultaneously.
+	ExecutionModeParallel ExecutionMode = "parallel"
+	// ExecutionModeSequential runs due jobs one after another, in order.
+	ExecutionModeSequential ExecutionMode = "sequential"
+)
+
+// OverlapPolicy decides what happens when a job's next run fires while the
+// previous run is still active.
+type OverlapPolicy string
+
+const (
+	// OverlapPolicySkip discards the new run when the job is already running.
+	OverlapPolicySkip OverlapPolicy = "skip"
+	// OverlapPolicyQueue holds the new run and starts it as soon as the current
+	// run finishes.
+	OverlapPolicyQueue OverlapPolicy = "queue"
+)
+
 // Config is stored in gosentry.json next to the program. It contains only
 // application-level choices: where to read jobs from, where to write logs, and
 // how the desktop shell should behave.
 type Config struct {
-	JobsDir           string `json:"jobs_dir"`
-	LogsDir           string `json:"logs_dir"`
-	MaxLogFiles       int    `json:"max_log_files"`
-	MaxLogAgeDays     int    `json:"max_log_age_days"`
-	StartOnLogin      bool   `json:"start_on_login,omitempty"`
-	KeepRunningInTray bool   `json:"keep_running_in_tray,omitempty"`
-	NotifyOnFailure   bool   `json:"notify_on_failure,omitempty"`
+	JobsDir           string        `json:"jobs_dir"`
+	LogsDir           string        `json:"logs_dir"`
+	MaxLogFiles       int           `json:"max_log_files"`
+	MaxLogAgeDays     int           `json:"max_log_age_days"`
+	StartOnLogin      bool          `json:"start_on_login,omitempty"`
+	KeepRunningInTray bool          `json:"keep_running_in_tray,omitempty"`
+	NotifyOnFailure   bool          `json:"notify_on_failure,omitempty"`
+	ExecutionMode     ExecutionMode `json:"execution_mode,omitempty"`
+	OverlapPolicy     OverlapPolicy `json:"overlap_policy,omitempty"`
 }
 
 // JobsFile is the on-disk shape of jobs.json. Wrapping the slice in a top-level