Session 0018.0 (Wave 9): rewrite legacy session refs in transcript bodies + README scoping

- Rewrite every legacy `Session L` / `Session M.1` etc. reference inside
  transcript bodies to the numeric form (`Session 0012.0` / `Session 0013.1`),
  via a mechanical Python sed pass scoped to canonical-filename transcripts
  only (502 replacements across 39 files; 21 files in this published repo +
  18 sibling files in `~/git/ohm-infra/`). The §23 rename was originally
  filename-only — session 0018.0 reverses that and rewrites the bodies too,
  so cross-references inside a transcript match the filename you'd browse
  to. The mapping table (`A → 0001` … `N → 0014`) lives in
  `SESSION-PROTOCOL.md` §1 (also amended this session) for anyone hitting
  an old external link.

- Amend README.md:
  - New "Where the record begins" section noting OHM started with two
    vibe-coded prototypes (no transcripts; the discipline that produced this
    record didn't exist yet). The transcripts here begin at the
    spec-driven-development phase — not at the literal beginning of OHM.
  - Update the existing "Sessions 0001 through 0013 originally shipped
    under a letter-form naming" paragraph to reflect that body refs are
    now also in numeric form.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

0007/SESSION-0007.0-TRANSCRIPT-2026-05-27T19-30--2026-05-27T19-45.md

@@ -1,4 +1,4 @@
-# Session G — Transcript
+# Session 0007.0 — Transcript
 
 > Date: 2026-05-27
 > Goal: Execute Slice 4 of the `ohm-rfc-app-flotilla` SPEC §20 slicing
@@ -154,7 +154,7 @@ $ git status
 (clean)
 ```
 
-Repo is clean from Session F's tail; nothing in flight to reconcile.
+Repo is clean from Session 0006.0's tail; nothing in flight to reconcile.
 
 **Codebase read**: `cli.py` (375 lines, Click groups for `deployment`,
 `overlay`, `secret`, `pin`, plus `deploy` with `--dry-run`-only),