docs: Phase B export improvements design

Covers B1 (summary block), B2 (custom step markers), B3 (detail levels),
B4 (editable preview). No migration needed.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/plans/2026-02-13-export-phase-b-design.md

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+# Export Improvements Phase B — Design
+
+> **Date:** 2026-02-13
+> **Depends on:** Phase A (complete on `feat/export-phase-a`)
+> **Scope:** B1 (Summary Block), B2 (Custom Step Markers), B3 (Detail Levels), B4 (Editable Preview)
+
+---
+
+## Decisions Made
+
+| Question | Decision |
+|----------|----------|
+| B1 summary form vs single editable preview | Single editable preview (B4). No separate structured form for summary fields. |
+| Detail levels | standard/full only. Dropped "summary" level — primary users are engineers, not dispatchers. |
+| Mid-session editable preview | No. TreeNavigationPage keeps quick one-click copy. Editable preview is SessionDetailPage only. |
+
+---
+
+## 1. Schema Changes
+
+Add to `SessionExport` (backend + frontend):
+
+```python
+include_summary: bool = False
+detail_level: Literal["standard", "full"] = "standard"
+```
+
+No database migration needed — these are export-time options only.
+
+---
+
+## 2. Custom Step Differentiation (B2)
+
+Detect custom steps by checking `node_id.startswith("custom-")` in each decision dict.
+
+**Markdown:**
+```markdown
+### Step 5: [CUSTOM] Check Additional Event Logs
+*Custom step added by engineer*
+```
+
+**Text:**
+```
+5. [CUSTOM] Check Additional Event Logs
+```
+
+**HTML:**
+```html
+<span style="background: #7c3aed; color: white; padding: 2px 6px; border-radius: 3px; font-size: 0.75em; margin-right: 6px;">CUSTOM</span>
+```
+
+**PSA:**
+```
+5. [CUSTOM] Check Additional Event Logs -> ...
+```
+
+Pure backend change — all 4 generators in `export_service.py`.
+
+---
+
+## 3. Summary Block (B1)
+
+When `include_summary=True`, insert a Summary section after metadata, before Evidence/Steps.
+
+Auto-populated fields:
+
+| Field | Source |
+|-------|--------|
+| Issue | Tree name + description |
+| Impact | `[Edit in preview]` placeholder |
+| Status | "Resolved" if completed, else "In Progress — paused at step N" |
+| Resolution | `outcome_notes` if available |
+| Next Steps | `next_steps` if available |
+
+Blank/placeholder fields are editable in the preview modal (B4). Format varies by generator (markdown table, text key-value, HTML styled table, PSA `--- SUMMARY ---` section).
+
+The summary block is opt-in (`include_summary=False` default), independent of detail level.
+
+---
+
+## 4. Detail Levels (B3)
+
+Two levels:
+
+- **standard** (default): Current behavior, except command outputs >5 lines are truncated with `*(full output omitted — N lines)*`
+- **full**: No truncation. All command outputs, scratchpad, notes rendered completely.
+
+Implementation: Helper function `_truncate_command_output(output, max_lines=5)` used in all 4 generators when `detail_level="standard"`.
+
+Frontend: Dropdown on SessionDetailPage export controls — "Standard" / "Full Detail".
+
+---
+
+## 5. Editable Preview (B4)
+
+Modify `ExportPreviewModal`:
+
+- Replace read-only `<pre>` with editable `<textarea>`
+- Local state `editedContent` initialized from `content` prop
+- "Copy" copies `editedContent`
+- "Download" downloads `editedContent`
+- "Reset" button restores original content
+- "Include Summary" checkbox re-fetches export with `include_summary=true`
+- Edits are NOT saved back to the session
+
+Only applies to SessionDetailPage. TreeNavigationPage keeps instant "Copy for Ticket" behavior.
+
+---
+
+## Files Affected
+
+**Backend:**
+- `backend/app/schemas/session.py` — Add `include_summary`, `detail_level` to `SessionExport`
+- `backend/app/services/export_service.py` — All 4 generators: custom step markers, summary block, command output truncation
+
+**Frontend:**
+- `frontend/src/types/session.ts` — Add `include_summary`, `detail_level` to `SessionExport`
+- `frontend/src/components/session/ExportPreviewModal.tsx` — Editable textarea, reset, summary toggle
+- `frontend/src/pages/SessionDetailPage.tsx` — Detail level dropdown, pass new options to export calls
+
+**No migration needed.**