chore(07-06): land the quick-task 261024-001 logo-asset replacement residue

- /start sendPhoto now points at MASTER_FILE-arah2_logo.png (deployed
  from web/public/ — the untracked asset lands here so the reference
  resolves on deploy)
- README + AGENTS.md + the 4 mockup screens swapped the Stitch
  googleusercontent logo placeholder for the local master logo
- pre-existing on the tree through 07-03..07-05 execution; verified by
  this session's baselines (bot 126/126, web 165/165, captures 22/22)

README.md

@@ -1,7 +1,7 @@
 <!-- generated-by: gsd-doc-writer -->
 
 <p align="center">
-  <img src="assets/logo_arah2.png" alt="arah² logo" width="140">
+  <img src="assets/MASTER_FILE-arah2_logo.svg" alt="arah² logo" width="140">
 </p>
 
 # arah²