bring the repo up to repo standards (closes #1)

Adopt the standard tooling from sneak/prompts: the Makefile and script/
entrypoints, lint and test stages in one Dockerfile, the canonical lint
config, the CI workflow and the formatter settings. Change the module
path to sneak.berlin/go/dcf.

Fix the lint findings in pkg/dcf in place. DCFStore becomes Store and
DCFObject becomes Object. One behaviour changes: an error listing mount
points is now returned, where a typo used to drop it.

Replace the placeholder test with real tests. Move the README into the
required sections. make build now compiles the library; it pointed at a
command that does not exist.

Model: opus-5-5

.gitignore

@@ -1 +1,32 @@
-dcfinfo
+# OS
+.DS_Store
+Thumbs.db
+
+# Editors
+*.swp
+*.swo
+*~
+*.bak
+.idea/
+.vscode/
+*.sublime-*
+
+# Agent scratch (worktrees of this repo, created and destroyed by
+# in-flight tooling). Unanchored: .gitignore patterns already match at
+# every depth, so no prefix is wanted here. This is not a .dockerignore
+# entry and must not be given a `**/` prefix on the way into one.
+.claude/
+
+# Node
+node_modules/
+
+# Go
+*.log
+*.out
+*.test
+
+# Environment / secrets
+.env
+.env.*
+*.pem
+*.key