Plugin: handover step + auto-seed prior notes into next session (hc-dy9)

Add a final "Notes for next time" step so the tech finishing today can flag
pending issues, watch-fors, and outstanding client decisions for whoever
picks up the next healthcheck on the same site.

On ATT_HC_Session::start() for a given site_key, the server's step history
for the handover step is queried (limit 1, excluding the just-created
session). If a prior session left handover notes, they're written into
the new session's "Before You Start" notes prefixed with the prior
session's date ("From previous session (YYYY-MM-DD):") so the carry-over
is obvious. The tech can edit/clear them as normal step notes from there.

- includes/steps/125-handover.php — new step (id=handover) using the
  standard notes field. No server schema or API change; it's just another
  step row in step_updates, surfaced like any other.
- ATT_HC_Session::seed_before_notes_from_prior_handover() — best-effort,
  silent degrade on API failure. The session is already registered on
  the server before this runs, so a failed seed never blocks start.
- No seed on resume() — resuming an existing session would clobber
  whatever the tech had already typed.

Verified end-to-end against the live MySQL server: handover-test-XXXX
flow shows carry-over with date prefix; no-handover-XXXX flow confirms
no false-positive seed for a fresh site_key. Test rows purged.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

includes/steps/125-handover.php

@@ -0,0 +1,25 @@
+<?php
+if (!defined('ABSPATH')) exit;
+
+/**
+ * Last step. Anything written in this step's notes is carried into the
+ * "Before You Start" notes of the NEXT session for the same site_key, so
+ * the next engagement starts with the prior tech's flags already visible.
+ *
+ * The carry-over happens in ATT_HC_Session::start() — see seed_before_notes_from_prior_handover().
+ */
+return new class extends ATT_HC_Step {
+    public function id(): string { return 'handover'; }
+    public function title(): string { return 'Notes for next time'; }
+    public function blurb(): string {
+        return 'Anything to flag for whoever picks this up at the next healthcheck — pending issues, things that nearly broke, gotchas, client decisions still outstanding. Whatever you write here will be pre-filled into the "Before You Start" step of the next session for this site, so it gets read.';
+    }
+    public function sub_items(): array {
+        return [
+            'Pending issues that didn\'t need fixing today but might next time',
+            'Decisions you\'re waiting on from the client',
+            'Things that nearly went wrong and what to watch for',
+            'Workarounds in place that should eventually be properly fixed',
+        ];
+    }
+};