refactor(tools): bin/{subject}/{method} layout; Go serve+watch modules

Move serve/ (module) -> bin/server, tools/ -> bin/tools, replace bin/kb-watch
bash with bin/watch Go package; self-executing Go shebangs bin/serve.go and
bin/kb/watch.go; Docker + CI + git/import + docs repointed. Multi-stage image
builds static serve+watch binaries (no Go runtime in container).

bin/watch/watch_test.go

@@ -0,0 +1,49 @@
+package watch
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+	"time"
+)
+
+func TestStampChangesWhenFileTouched(t *testing.T) {
+	dir := t.TempDir()
+	a := filepath.Join(dir, "a.md")
+	if err := os.WriteFile(a, []byte("x"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	s1 := Stamp([]string{dir})
+	if s1 == "" {
+		t.Fatal("stamp empty for a dir with a file")
+	}
+	time.Sleep(10 * time.Millisecond)
+	if err := os.WriteFile(a, []byte("y"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	if s2 := Stamp([]string{dir}); s2 == s1 {
+		t.Fatal("stamp did not change after the file was modified")
+	}
+}
+
+func TestStampEmptyForMissingDir(t *testing.T) {
+	if s := Stamp([]string{filepath.Join(t.TempDir(), "nope")}); s != "" {
+		t.Fatalf("stamp = %q, want empty for missing dir", s)
+	}
+}
+
+func TestFromEnvDefaults(t *testing.T) {
+	t.Setenv("KB_WATCH_INTERVAL", "")
+	t.Setenv("KB_WATCH_DIRS", "")
+	t.Setenv("KB_PY", "")
+	opts := fromEnv(nil)
+	if len(opts.Dirs) == 0 || opts.Dirs[0] != "/corpus" {
+		t.Fatalf("default dirs = %v, want [/corpus]", opts.Dirs)
+	}
+	if opts.Interval != 30*time.Second {
+		t.Fatalf("default interval = %s, want 30s", opts.Interval)
+	}
+	if opts.IndexCmd == "" {
+		t.Fatal("default index cmd is empty")
+	}
+}