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

.editorconfig

@@ -0,0 +1,12 @@
+root = true
+
+[*]
+indent_style = space
+indent_size = 4
+end_of_line = lf
+charset = utf-8
+trim_trailing_whitespace = true
+insert_final_newline = true
+
+[Makefile]
+indent_style = tab