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_test.go

@@ -1741,6 +1741,43 @@ func TestAgent_SSHConnectionLoginVars(t *testing.T) {
 	}
 }
 
+// TestAgent_SSHEnvInfoShell verifies that an agent.Options.EnvInfo whose
+// Shell() reports a custom shell is piped through to the SSH session, so the
+// session command runs under that shell instead of the host default.
+func TestAgent_SSHEnvInfoShell(t *testing.T) {
+	t.Parallel()
+	if runtime.GOOS == "windows" {
+		t.Skip("the fake shell is a POSIX script")
+	}
+
+	// A fake shell that ignores its arguments and prints a sentinel. The
+	// sentinel only appears in the session output if the injected Shell() was
+	// honored. Otherwise the command's own output ("should-not-run") appears.
+	const marker = "injected-shell-was-used"
+	shellPath := filepath.Join(t.TempDir(), "fakeshell")
+	//nolint:gosec // Executable test shell with test-controlled content.
+	err := os.WriteFile(shellPath, []byte("#!/bin/sh\necho "+marker+"\n"), 0o700)
+	require.NoError(t, err)
+
+	session := setupSSHSession(t, agentsdk.Manifest{}, codersdk.ServiceBannerConfig{}, nil, func(_ *agenttest.Client, o *agent.Options) {
+		o.EnvInfo = shellOverrideEnvInfo{shell: shellPath}
+	})
+
+	output, err := session.Output("echo should-not-run")
+	require.NoError(t, err)
+	require.Contains(t, string(output), marker)
+	require.NotContains(t, string(output), "should-not-run")
+}
+
+// shellOverrideEnvInfo is a usershell.EnvInfoer that delegates to the system
+// implementation but reports a custom shell.
+type shellOverrideEnvInfo struct {
+	usershell.SystemEnvInfo
+	shell string
+}
+
+func (e shellOverrideEnvInfo) Shell(string) (string, error) { return e.shell, nil }
+
 func TestAgent_Metadata(t *testing.T) {
 	t.Parallel()