Drop "prototype/carryover" framing now that v1 is shipped

SPEC, DEV docs, and code comments still talked about the codebase as
a rewrite-in-progress against an external prototype. With v1 shipped
the framing reads oddly — it implies code is provisional when it's
the production thing. Recast §18 as "the technical stack," strip
"carryover from the prototype" comments across backend (api.py,
chat.py, providers.py) and frontend (DiffView, PromptBar,
SelectionTooltip, modelStyles), and rework SPEC §1 / §18 to introduce
OHM up front rather than as a follow-on to a prototype reference.

Also:
- RUNBOOK: bump Python prereq to 3.11+ to match the production VM
  (was 3.13).
- Remove IMPLEMENTATION-PROMPT.md — the original implementation brief
  is no longer load-bearing.
- Add deploy/DEPLOY-NEW-SESSION-PROMPT.md as the durable
  deploy-handoff prompt for new sessions.

frontend/src/modelStyles.js

@@ -1,6 +1,6 @@
 // modelStyles.js — colors and display labels for each LLM provider.
-// §18 carryover from the prototype. Per §19.2's per-RFC-model topic,
-// future per-RFC overrides land on top of this map without replacing it.
+// Per §19.2's per-RFC-model topic, future per-RFC overrides land on
+// top of this map without replacing it.
 
 export const MODEL_STYLES = {
   // Claude variants — shades of purple