docs: add S5 screenshot and replace placeholder in getting-started guide

Mark the Model manager TUI screenshot task as complete in TODO.md and integrate the actual screenshot into the getting-started documentation, replacing the placeholder.

Author: nmaguiar

TODO.md

@@ -45,7 +45,7 @@ All screenshot placeholders in the documentation site. Replace `[SCREENSHOT-PLAC
 - [x] S2 — Console interactive goal
 - [x] S3 — Terminal quick start recording
 - [x] S4 — Architecture diagram
-- [ ] S5 — Model manager TUI
+- [x] S5 — Model manager TUI
 - [ ] S6 — Console first-run
 - [ ] S7 — Web UI first-run
 - [ ] S8 — Dual-model cost stats

assets/images/screenshots/s5-model-manager-tui.png

@@ -90,7 +90,7 @@ mini-a includes a built-in TUI (text user interface) for managing models interac
 mini-a modelman=true
 ```
 
-<div class="screenshot-placeholder">[SCREENSHOT-PLACEHOLDER: S5 — Model manager TUI]</div>
+<img src="{{ '/assets/images/screenshots/s5-model-manager-tui.png' | relative_url }}" alt="Model manager TUI screen" style="border-radius:8px; border:1px solid rgba(160,174,192,0.3);">
 
 ---