Add .editorconfig, broaden .gitignore, drop dead Drone refs (closes #72)

Add the canonical .editorconfig verbatim from sneak/prompts. Merge secret
(.env, .env.*, *.key, *.pem), OS (.DS_Store, Thumbs.db), editor (*.swp,
*.swo, *~, .idea/, .vscode/), and Go (*.log, *.out, *.test) patterns into
.gitignore while preserving the repo-specific entries. Remove the stale
.drone.yml ignore entry and the DRONE_COMMIT_SHA branch from bin/gitrev.sh,
left over from the Gitea Actions migration.

bin/gitrev.sh remains correct: the GITREV env path (used in the Docker
build) and the git describe fallback (Gitea Actions and local developers)
both still produce output. bin/gitrev.sh stays tracked despite /bin/, and
git status --ignored confirms no tracked file became ignored.

The canonical .editorconfig sets space/4 for [*]; Go files are tab-indented
via gofmt, which editorconfig does not govern. Taken verbatim per the issue.

Model: opus-4-8

TODO.md

@@ -24,6 +24,9 @@ only thing left of the `chore/align-repo-policies` branch is the list below.
 
 # Completed Steps
 
+- 2026-09-21: added the canonical `.editorconfig`, made `.gitignore` cover
+  secrets, OS, editor, and Go artifacts, and removed the dead Drone CI
+  references from `.gitignore` and `bin/gitrev.sh` (#72)
 - 2026-08-09: added `.prettierrc`/`.prettierignore`, gave `script/fmt` and
   `script/fmt-check` one shared prettier file set via `script/prettier`, dropped
   the `|| true` that hid prettier failures, and added a node-based Dockerfile