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.

deploy/RUNBOOK.md

@@ -20,9 +20,10 @@ recover from a partial install is safe.
   `git.wiggleverse.org` over HTTPS.
 - DNS: an `A` record for `rfc.wiggleverse.org` pointing at the same IP as
   `git.wiggleverse.org`.
-- Python 3.13 available system-wide. Node 20+ available (for `npm run
-  build` once; the build output is what runs in production — Node isn't
-  needed at runtime).
+- Python 3.11+ available system-wide (the project has no `requires-python`
+  pin; the current production VM runs 3.11 on Debian bookworm). Node 20+
+  available (for `npm run build` once; the build output is what runs in
+  production — Node isn't needed at runtime).
 - `git`, `openssl`, and `rsync` on the host.
 
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