P2-05: forget command with retention policy

End-to-end forget plumbing — operator can create a forget schedule
with keep-* values, agent runs restic forget --keep-* … on the
schedule's cron (or via per-row Run-now), snapshot list shrinks,
UI updates.

* api.CommandRunPayload gains retention_policy json.RawMessage so
  the agent doesn't need a typed copy of the server-side struct.
* restic.ForgetPolicy mirrors restic's --keep-* flags. Empty()
  reports zero dimensions; restic wrapper RunForget refuses to
  run an empty policy (would delete every snapshot). Does NOT
  pass --prune — pruning lives behind a separate admin-only
  credential (P2-06); forget just rewrites the snapshot index.
* runner.RunForget mirrors RunBackup's envelope shape so the
  live log viewer works without special-casing. On success
  triggers reportSnapshots (forget shrinks the index, the host's
  snapshot count almost certainly changed).
* cmd/agent dispatcher handles MsgCommandRun with kind=forget,
  decodes RetentionPolicy from the wire, builds restic.ForgetPolicy.
* Server dispatchScheduleNow marshals the schedule's
  RetentionPolicy into the wire payload for kind=forget jobs.
  Refuses to dispatch a forget schedule with empty retention.
* validateSchedule rejects kind=forget without at least one keep-*
  dimension (new error code: missing_retention).
* UI schedule edit form gains a Kind dropdown (backup or forget;
  immutable on edit). Paths block toggles by kind via inline
  data-kind attributes. Form help-text explains the prune
  separation.

Other kinds (prune, check, unlock) deferred to P2-06..08; the
Kind dropdown only offers backup and forget today.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/api/messages.go

@@ -66,10 +66,14 @@ const (
 )
 
 // CommandRunPayload is the server → agent dispatch for a run-now job.
+// RetentionPolicy is populated for kind=forget jobs (raw JSON so the
+// agent doesn't need to share the typed struct definition with the
+// server's store package).
 type CommandRunPayload struct {
-	JobID string   `json:"job_id"`
-	Kind  JobKind  `json:"kind"`
-	Args  []string `json:"args,omitempty"`
+	JobID           string          `json:"job_id"`
+	Kind            JobKind         `json:"kind"`
+	Args            []string        `json:"args,omitempty"`
+	RetentionPolicy json.RawMessage `json:"retention_policy,omitempty"`
 }
 
 // CommandCancelPayload is the server → agent cancel signal.