bring the repo up to org standards

Adopt the standard tooling: a Makefile of thin shims over a full
scripts-to-rule-them-all `script/` set, the canonical `.golangci.yml`
vendored byte-identical from `sneak/prompts`, docker-only linting via
`Dockerfile.lint`, a `Dockerfile` and Gitea workflow that gate every
push, and prettier/editorconfig/dockerignore config.

`make build` pointed at a `cmd/dcfinfo` that is not in the tree and
could never have succeeded; this repo is a library, so `build` is now
the compile check over every package.

Clear the 57 findings the canonical linter config reports on `pkg/dcf`.
Two were real: `findDCFMountPoints` checked a never-assigned `erro`
instead of the error from `findAllMountPoints`, discarding it, and
`privatePath` was computed twice so the first computation was dead.
Mountpoint selection otherwise behaves exactly as before; the defects
that survive are filed as issue #6, not fixed here.

Rename `DCFStore` to `Store` and `DCFObject` to `Object` (with its
`DCFStoreRoot` field to `StoreRoot`), which revive's stutter rule
requires and which the `fs.FS` rework in issue #4 will build on.

Replace the placeholder test with tests over the exported surface.
The filesystem walk stays uncovered: it is reachable only through
`GetDCFStores`, which needs real mounted media.

README keeps its content, reorganised into the required sections and
gaining Entrypoints.

(closes #1)

.dockerignore

@@ -0,0 +1,20 @@
+# Never exclude Go sources, go.mod, go.sum or .golangci.yml: the lint
+# and test stages only ever examine what reaches the build context, and
+# an exclusion here makes them pass over a tree that is missing a
+# package. script/assert-context-complete exists to catch exactly that,
+# and will fail the build rather than let it happen silently.
+.git/
+.gitea/
+README.md
+LICENSE
+.editorconfig
+.prettierrc
+.prettierignore
+.env
+.env.*
+.DS_Store
+.idea/
+.vscode/
+.claude/
+tmp/
+temp/