feat(agent): unify session env via EnvInfoer (#26099)

The agent resolved the session home directory two ways. agentssh went
through usershell, while agentproc called os.UserHomeDir directly and
skipped its user.Current fallback. Routing both through a single
usershell.EnvInfoer makes the resolution consistent, and agentproc
now gets the same fallback as the rest of the agent.

The shared seam is injectable, so SSH session tests can drive
environment resolution without touching real system state.

agent/agent.go

@@ -52,6 +52,7 @@ import (
 	"github.com/coder/coder/v2/agent/proto"
 	"github.com/coder/coder/v2/agent/proto/resourcesmonitor"
 	"github.com/coder/coder/v2/agent/reconnectingpty"
+	"github.com/coder/coder/v2/agent/usershell"
 	"github.com/coder/coder/v2/agent/x/agentdesktop"
 	"github.com/coder/coder/v2/agent/x/agentmcp"
 	"github.com/coder/coder/v2/buildinfo"
@@ -89,7 +90,10 @@ type Options struct {
 	Client                 Client
 	ReconnectingPTYTimeout time.Duration
 	EnvironmentVariables   map[string]string
-	Logger                 slog.Logger
+	// EnvInfo overrides the session command environment source. Only
+	// tests set this. Nil defaults to usershell.SystemEnvInfo.
+	EnvInfo usershell.EnvInfoer
+	Logger  slog.Logger
 	// IgnorePorts tells the api handler which ports to ignore when
 	// listing all listening ports. This is helpful to hide ports that
 	// are used by the agent, that the user does not care about.
@@ -226,6 +230,7 @@ func New(options Options) Agent {
 		statsReportInterval:                options.StatsReportInterval,
 		announcementBannersRefreshInterval: options.ServiceBannerRefreshInterval,
 		sshMaxTimeout:                      options.SSHMaxTimeout,
+		envInfo:                            options.EnvInfo,
 		subsystems:                         options.Subsystems,
 		logSender:                          agentsdk.NewLogSender(options.Logger),
 		blockFileTransfer:                  options.BlockFileTransfer,
@@ -303,6 +308,7 @@ type agent struct {
 	announcementBannersRefreshInterval time.Duration
 	sshServer                          *agentssh.Server
 	sshMaxTimeout                      time.Duration
+	envInfo                            usershell.EnvInfoer
 	blockFileTransfer                  bool
 	blockReversePortForwarding         bool
 	blockLocalPortForwarding           bool
@@ -365,6 +371,7 @@ func (a *agent) init() {
 		AnnouncementBanners:        func() *[]codersdk.BannerConfig { return a.announcementBanners.Load() },
 		UpdateEnv:                  a.updateCommandEnv,
 		WorkingDirectory:           func() string { return a.manifest.Load().Directory },
+		EnvInfo:                    a.envInfo,
 		BlockFileTransfer:          a.blockFileTransfer,
 		BlockReversePortForwarding: a.blockReversePortForwarding,
 		BlockLocalPortForwarding:   a.blockLocalPortForwarding,
@@ -420,7 +427,7 @@ func (a *agent) init() {
 
 	pathStore := agentgit.NewPathStore()
 	a.filesAPI = agentfiles.NewAPI(a.logger.Named("files"), a.filesystem, pathStore)
-	a.processAPI = agentproc.NewAPI(a.logger.Named("processes"), a.execer, a.updateCommandEnv, pathStore, func() string {
+	a.processAPI = agentproc.NewAPI(a.logger.Named("processes"), a.execer, pathStore, a.envInfo, a.updateCommandEnv, func() string {
 		if m := a.manifest.Load(); m != nil {
 			return m.Directory
 		}