feat: add procedural flows with intake forms, navigation, and seed templates

Adds a new "procedural" tree type for linear step-by-step project workflows
(domain controller setup, M365 onboarding, VPN config, etc). Includes intake
form builder, two-panel step navigation, variable resolution, procedural
exports, 3 seed templates, and UI rename from "Trees" to "Flows".

Also archives 19 implemented plan docs and creates deferred features backlog.

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

LESSONS-LEARNED.md

@@ -890,6 +890,39 @@ def _fix_node_fields(node):
 
 ---
 
+## Export / Redaction
+
+### CORS `expose_headers` Required for Custom Response Headers
+**Problem:** Frontend reads custom response headers (e.g. `X-Redaction-Summary`) but gets `undefined` — the header exists in the response but Axios can't access it.
+
+**Cause:** Browsers enforce CORS restrictions on response headers. Only "CORS-safelisted" headers (Cache-Control, Content-Type, etc.) are accessible by default. Custom headers require explicit exposure.
+
+**Solution:** Add `expose_headers` to CORS middleware in `main.py` (both branches):
+```python
+app.add_middleware(
+    CORSMiddleware,
+    allow_origins=settings.allowed_origins,
+    expose_headers=["X-Redaction-Mode", "X-Redaction-Summary"],
+    ...
+)
+```
+
+**Files affected:** `backend/app/main.py` (both CORS middleware branches)
+
+---
+
+### Redaction Must Run AFTER Variable Resolution
+**Problem:** Sensitive data injected via session variables (e.g. `{{client_ip}}` → `192.168.1.1`) would bypass redaction if it ran before variable substitution.
+
+**Solution:** Export pipeline order matters:
+1. Generate export by format (markdown/html/text/psa)
+2. Resolve session variables
+3. Apply redaction (if `redaction_mode == "mask"`)
+
+**Key file:** `backend/app/api/endpoints/sessions.py` — the redaction block is placed after both generation and variable resolution, with fail-closed error handling (500 on failure, never return unredacted content).
+
+---
+
 ## Adding New Lessons
 
 When you encounter and fix a bug, add it here with: