Add background process execution API

Start long-running processes (web servers, daemons) without blocking the
HTTP request. Leverages envd's existing background process support
(context.Background(), List, Connect, SendSignal RPCs) and wires it
through the host agent and control plane layers.

New API surface:
- POST /v1/capsules/{id}/exec with background:true → 202 {pid, tag}
- GET /v1/capsules/{id}/processes → list running processes
- DELETE /v1/capsules/{id}/processes/{selector} → kill by PID or tag
- WS /v1/capsules/{id}/processes/{selector}/stream → reconnect to output

The {selector} param auto-detects: numeric = PID, string = tag.
Tags are auto-generated as "proc-" + 8 hex chars if not provided.

internal/api/server.go

@@ -74,6 +74,7 @@ func New(
 	buildH := newBuildHandler(buildSvc, queries, pool)
 	channelH := newChannelHandler(channelSvc, al)
 	ptyH := newPtyHandler(queries, pool)
+	processH := newProcessHandler(queries, pool)
 	adminCapsules := newAdminCapsuleHandler(sandboxSvc, queries, pool, al)
 
 	// OpenAPI spec and docs.
@@ -141,6 +142,9 @@ func New(
 			r.Post("/files/remove", fsH.Remove)
 			r.Get("/metrics", metricsH.GetMetrics)
 			r.Get("/pty", ptyH.PtySession)
+			r.Get("/processes", processH.ListProcesses)
+			r.Delete("/processes/{selector}", processH.KillProcess)
+			r.Get("/processes/{selector}/stream", processH.ConnectProcess)
 		})
 	})
 
@@ -224,6 +228,9 @@ func New(
 			r.Post("/files/remove", fsH.Remove)
 			r.Get("/metrics", metricsH.GetMetrics)
 			r.Get("/pty", ptyH.PtySession)
+			r.Get("/processes", processH.ListProcesses)
+			r.Delete("/processes/{selector}", processH.KillProcess)
+			r.Get("/processes/{selector}/stream", processH.ConnectProcess)
 		})
 	})