docs(architecture): document the AI Career Assistant (Phase 5)

ai-career-assistant.md (modules, prompt flow, provider abstraction, append-only
history model, extension points, security). Master guide + roadmap Phase 5
updated with the shipped workspace and the open provider-selection extension.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/MASTER_IMPLEMENTATION_GUIDE.md

@@ -56,6 +56,10 @@ career data is stored and how generated documents reference it.
 - `docs/cv-builder-product-teardown.md` — product teardown.
 - `docs/career-workspace-branch-assessment.md` — per-table recover/migrate/replace verdict
   for the recovered Career/CV tables.
+- `docs/architecture/cv-builder.md` + `docs/architecture/cv-theme-engine.md` — the shipped CV Builder
+  (Phase 4/4.5): variant lens over the master profile, data-driven theme engine, public CV routing.
+- `docs/architecture/ai-career-assistant.md` — the shipped AI Workspace (Phase 5): per-application
+  suggestion modules, append-only history, provider abstraction, extension points.
 
 **Non-negotiable, restated from those documents:** the master career profile is the single
 source of truth. Career data is never duplicated into CVs, applications, or variants — a