Clarify session startup flow after README front door

SESSION_START.md

@@ -1,14 +1,31 @@
 # Session Start Checklist
 
-Before working:
+`README.md` is the front door.
 
-1. Review OPEN_THREADS
+After reading `README.md`, use this checklist to start work.
-2. Review CONSTRAINTS
-3. Confirm no drift between:
-   - Portal
-   - API
-   - Agent
-4. Confirm file model assumptions are accurate
-5. Confirm upload transport is using raw piping
 
-Never assume speculative features are implemented.
+## Required
+
+1. Review `OPEN_THREADS.md`
+2. Review `CONSTRAINTS.md`
+
+## Read If Relevant
+
+3. Review `PROJECT_CONTEXT.md` if current platform state or routing model matters
+4. Review `INFRASTRUCTURE.md` if VM, IP, or host context matters
+5. Review `UPSTREAMS.md` if other repos or external dependencies matter
+
+## Deep Recovery Only
+
+Use these only if the live working set is not enough:
+
+- `SESSION_LOG/`
+- `Session_Summaries/`
+- `SCRATCH/`
+
+## Final Check Before Working
+
+- Confirm no drift between portal, API, and agent assumptions
+- Confirm file model assumptions are accurate
+- Confirm upload transport is using raw piping
+- Never assume speculative features are implemented