docs: add project documentation and task tracking

Why:
- Need to track domain model, ADRs, task progress, and feature planning

Changes:
- CONTEXT.md: domain model with ADRs 0001-0014
- docs/tasks.md: updated task list with Phase 6 (Dashboard)
- docs/cant-do-yet.md: backend-ready but no UI features
- docs/out-of-scope-v1.md: intentionally excluded features

Impact:
- Project documentation centralized for reference

docs/cant-do-yet.md

@@ -0,0 +1,53 @@
+# What You Can't Do Yet (But Backend Is Ready)
+
+Features where the backend API is implemented and tested, but the dashboard UI does not yet expose them.
+
+## 1. Chunk Preview Panel (Admin Tab)
+
+**Backend**: `GET /admin/chunks/{doc_id}?strategy=...` returns all chunks for a document and strategy — chunk text (truncated to 500 chars), chunk index, token count, character count, parent_id.
+
+**Missing UI**: No panel in the Admin tab to select a document + strategy and visualize the actual chunk text blocks, their sizes, and hierarchy.
+
+## 2. Questions Dataset Management (Admin Tab)
+
+**Backend**:
+- `GET /admin/questions` — lists JSON files in `files/` with question counts
+- `POST /admin/questions/upload` — uploads a new questions JSON file
+- `GET /admin/questions/{file_id}` — returns full question set
+- `DELETE /admin/questions/{file_id}` — deletes a question file
+
+**Missing UI**: No section in the Admin tab to browse, upload, view, or delete question datasets. You can only use the default `files/questions.json` by typing its path in the Benchmarks tab.
+
+## 3. Standalone Cost Estimator (Admin Tab)
+
+**Backend**: `POST /admin/cost-estimate` accepts `num_questions` and `num_strategies`, returns full cost breakdown (embedding, queries, evaluation, token estimates, total USD).
+
+**Missing UI**: No standalone cost estimation panel in the Admin tab. The Benchmarks tab has a "Cost Estimate" button, but it is tied to the benchmark form and requires a document selection.
+
+## 4. Experiment Detail View (Benchmarks Tab)
+
+**Backend**: `GET /benchmarks/{experiment_id}` returns full experiment data — per-question results, per-strategy scores, aggregate metrics, benchmark config.
+
+**Missing UI**: Dashboard shows a list of experiments with a "Report" link that opens the raw HTML report in a new tab. No in-dashboard detail view with side-by-side strategy comparison tables, per-question drill-down, or visual charts.
+
+## 5. Query History (Query Tab)
+
+**Backend**: `GET /queries/{query_id}` retrieves any past query. All queries are stored in SQLite with full results (answer, retrieved chunks, latency, tokens).
+
+**Missing UI**: No list of past queries. When you navigate away from the Query tab, the current result disappears from the screen. There is no history panel to browse, search, or re-view previous questions and answers.
+
+## 6. Document Content Preview (Documents Tab)
+
+**Backend**: `GET /documents` returns `parsed_text_preview` and `document_tree` fields in the detail response model (`DocumentDetailResponse`). SQLite stores the full parsed text and document tree.
+
+**Missing UI**: You can upload and process documents, but you cannot preview the actual document text or the hierarchical document tree (sections, articles, paragraphs) inside the dashboard.
+
+## 7. Real-Time Processing Status (Documents Tab)
+
+**Backend**: The processing endpoint (`POST /documents/{id}/process`) returns per-strategy results — `strategies_completed` and `strategies_failed` with status, chunk counts, and error messages.
+
+**Missing UI**: When you click "Process", the button shows a spinner. You don't see which strategies are running, which have completed, or which failed. If it fails partway through 5 strategies, you don't know which one broke without checking the server logs.
+
+## 8. Global Error Handling and Retry
+
+**Missing**: If the server returns an error (network down, Qdrant disconnected, OpenAI rate limit), you see a toast notification that disappears after 3 seconds. There is no retry button, no persistent error banner, and no automatic reconnection. A failed operation must be manually re-attempted by clicking the button again.