servicepack v1.2.19 — build-context and ignore-file hygiene

No code change.

.dockerignore was a single line while every Dockerfile does `COPY . .`, and the
dev Dockerfiles END at that COPY -- so the local release script git-update.sh
was shipping inside the dev images. The file now covers git/CI metadata, root
and docs/ documentation, env files, local tooling, build output, coverage, logs,
editor state, backups, research scratch and .agents.

Kept on purpose, with the reasoning written into the file: vendor/ (-mod=vendor),
*_test.go (the dev image runs make test), and nested markdown, since a package
may go:embed markdown as a resource. Only root-level and docs/ markdown is
dropped.

Also fixes .research_files never having been gitignored -- the entry was missing
its leading dot.

Neither file propagates to existing projects; copy the entries by hand.