Fix review round 1 of the wait list and its hooks (issues 1629, 1743, 1744)

A session id reaching the Stop and SessionStart hooks was only checked
for blank, so "../../x" or "a/b" could read or write outside the waits
directory; both hooks now hold every session id to waitSessionID before
building any path, and allow (saying why) rather than fail on a bad one.
claude_code_wait's list/watched/done/said held only add to that same
check, letting a malformed id through as an ordinary, always-empty key;
they all refuse it now.

The Stop hook's three-strikes cap could be bypassed forever if its
counter file could not be written on a refusal: it now allows instead,
saying so, so a wait can never hold a turn open indefinitely. Its advice
offered mesh_wait on a wait's event family unconditionally, ignoring
that the session's tool runner may not be granted it (ADR 0367 §2); it
now offers mesh_wait only "if this session's tool runner is granted it,
otherwise start a watcher", and drops the family entirely when a wait
has none.

Both hooks' fail-open and let-through notices were said only on stderr,
which Claude Code does not show the operator on an allowed (exit 0)
outcome; they now also print the same words as a "systemMessage" JSON
object on stdout, which Claude Code does show.

modules/claude-code/cmd/claude-code/session_start_test.go

@@ -4,6 +4,8 @@ package main
 
 import (
 	"bytes"
+	"fmt"
+	"os"
 	"path/filepath"
 	"strings"
 	"testing"
@@ -72,6 +74,45 @@ type panicReader struct{}
 
 func (panicReader) Read([]byte) (int, error) { panic("boom") }
 
+// Review round 1, item 1 (1743/1744, ADR 0489 §5): a session id that is not a plain one is never used to
+// build a file path — reading outside the waits dir never happens — and the hook prints only a line saying
+// the id could not be used.
+func TestSessionStartRejectsAnUnsafeSessionID(t *testing.T) {
+	root := t.TempDir()
+	waitsDir := filepath.Join(root, "agent", "waits")
+	if err := os.MkdirAll(waitsDir, 0o700); err != nil {
+		t.Fatal(err)
+	}
+	// What "../../x" resolves to from waitsDir: root/x.json, outside waitsDir, with content that must never be
+	// read through it.
+	outside := filepath.Join(waitsDir, "../../x.json")
+	writeWaitsFixture(t, outside, WaitsFile{Session: "x", At: time.Now(),
+		Waits: []Wait{{ID: "w-secret", What: "SECRET-DATA", Bound: 10, At: time.Now()}}})
+	t.Cleanup(func() { _ = os.Remove(outside) })
+
+	for _, id := range []string{"../../x", "a/b"} {
+		var out bytes.Buffer
+		stdin := strings.NewReader(fmt.Sprintf(`{"session_id":%q}`, id))
+		if code := runSessionStart(waitsDir, stdin, &out); code != 0 {
+			t.Fatalf("id %q: code %d", id, code)
+		}
+		if strings.Contains(out.String(), "SECRET-DATA") {
+			t.Fatalf("id %q: read a file outside the waits dir: %s", id, out.String())
+		}
+		if !strings.Contains(out.String(), "could not be used") {
+			t.Fatalf("id %q: did not say the id could not be used: %s", id, out.String())
+		}
+		// Item (a): the notice reaches the screen as the one JSON object Claude Code shows.
+		if !strings.Contains(out.String(), "systemMessage") {
+			t.Fatalf("id %q: the notice is not a systemMessage: %s", id, out.String())
+		}
+	}
+	var out bytes.Buffer
+	if code := runSessionStart(waitsDir, strings.NewReader(`{"session_id":""}`), &out); code != 0 || out.Len() != 0 {
+		t.Fatalf("blank id: code %d, out %q", code, out.String())
+	}
+}
+
 // 1744/AC4 is checked together with 1743/AC7 in terminal_only_test.go's
 // TestThePluginsOnlyHooksAreTheGuardTheSessionStartHookAndTheStopHook.