chore: drop the giant gitignore comment from #25 (post-merge nit)

[dgokeeffe]

Summary

Follow-up to #25, addressing @mpkrass7's review suggestion that landed after #25 merged. Drops the migration-context comment block in .gitignore — context belongs in PR/issue history, not in the codebase.

Why a separate PR

#25 merged at 2026-05-06T12:15 UTC, and Marshall's "LGTM, suggest removing the comment" came in at 12:12 — but the merge button got hit before the suggestion was applied. This is the cleanup.

Test

$ git check-ignore .codex/databricks-models.json && echo IGNORED || echo TRACKED
TRACKED                                                       # ✓
$ for f in .codex/config.toml .codex/.env .codex/AGENTS.md .codex/memories/x .codex/tmp/y; do
    git check-ignore "$f" >/dev/null && echo "$f → ignored" || echo "$f → leaked"
  done
.codex/config.toml → ignored                                   # ✓
.codex/.env → ignored                                          # ✓
.codex/AGENTS.md → ignored                                     # ✓
.codex/memories/x → ignored                                    # ✓
.codex/tmp/y → ignored                                         # ✓

Same set of ignored files as before #25, just less narrative in the file.

This pull request and its description were written by Isaac.