Add the plugin's SessionStart hook so a session learns its own id (issue 1744)

A session is never told its session id by any tool, only by a hook's
stdin, so it cannot name itself to claude_code_wait. The plugin's
SessionStart hook (claude-code session-start <waits-dir>) prints the id
and how to give it, plus the session's still-open waits after a /clear,
reading the waits file claude_code_wait mirrors directly rather than
calling back into this module. Carries the Stop hook's command and the
GuardFiles.WaitsDir plumbing MarketplaceWith needs for both (issue 1743
wires the Stop hook's dispatch next).

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

@@ -278,7 +278,7 @@ func guardFiles(p Paths, m *MeshTools) *GuardFiles {
 	if resolved, err := filepath.EvalSymlinks(exe); err == nil {
 		exe = resolved
 	}
-	return &GuardFiles{Exe: exe, Data: GuardDataOf(p, m)}
+	return &GuardFiles{Exe: exe, Data: GuardDataOf(p, m), WaitsDir: WaitsDirPath(p)}
 }
 
 // MeshToolsEvery is how often the controller is asked again.