feat(Phase4): add file changes for changestats executor wiring

Files changed: CLAUDE.md, internal/api/changestats.go,
internal/executor/executor.go, internal/executor/executor_test.go,
internal/task/changestats.go (new)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

1 files changed, 47 insertions, 0 deletions

diff --git a/internal/task/changestats.go b/internal/task/changestats.go
new file mode 100644
index 0000000..95be8ec
--- /dev/null
+++ b/internal/task/changestats.go
@@ -0,0 +1,47 @@
+package task
+
+import (
+	"bufio"
+	"os"
+	"regexp"
+	"strconv"
+	"strings"
+)
+
+// gitDiffStatRe matches git diff --stat summary lines, e.g.:
+//
+//	"3 files changed, 50 insertions(+), 10 deletions(-)"
+//	"1 file changed, 5 insertions(+)"
+//	"1 file changed, 3 deletions(-)"
+var gitDiffStatRe = regexp.MustCompile(`(\d+) files? changed(?:, (\d+) insertions?\(\+\))?(?:, (\d+) deletions?\(-\))?`)
+
+// ParseChangestatFromOutput scans text for git diff --stat summary lines and
+// returns the first match as a Changestats value. Returns nil if no match found.
+func ParseChangestatFromOutput(output string) *Changestats {
+	scanner := bufio.NewScanner(strings.NewReader(output))
+	for scanner.Scan() {
+		line := scanner.Text()
+		if m := gitDiffStatRe.FindStringSubmatch(line); m != nil {
+			cs := &Changestats{}
+			cs.FilesChanged, _ = strconv.Atoi(m[1])
+			if m[2] != "" {
+				cs.LinesAdded, _ = strconv.Atoi(m[2])
+			}
+			if m[3] != "" {
+				cs.LinesRemoved, _ = strconv.Atoi(m[3])
+			}
+			return cs
+		}
+	}
+	return nil
+}
+
+// ParseChangestatFromFile reads a log file and extracts the first git diff stat
+// summary it finds. Returns nil if the file cannot be read or contains no match.
+func ParseChangestatFromFile(path string) *Changestats {
+	data, err := os.ReadFile(path)
+	if err != nil {
+		return nil
+	}
+	return ParseChangestatFromOutput(string(data))
+}