Install from @latest, retract v0.0.1, print a go-installed binary's version (closes #116)

README "Install" now leads with go install ...@latest: until 1.0 each
release is a v0.0.N tag on main's tip, so @latest gives that tip once
it has passed the build. It also says what @main and v0.0.1 give and
how to check an installed binary. go.mod retracts v0.0.1, the early
build without a database.

sfdupes --version printed "dev" for every go-installed binary, since
only the linker set Version. When the linker sets none, it now prints
the main module's version from runtime/debug.ReadBuildInfo; "dev" is
left only when neither is known. Version is now empty by default.

The retraction takes effect only once a release carrying it is the
newest.

Model: opus-5-5

TODO.md

@@ -29,6 +29,16 @@
 
 # Completed Steps
 
+- releases until 1.0 are `v0.0.N` tags on `main`'s tip, so
+  `go install sneak.berlin/go/sfdupes@latest` installs `main`'s tip once it has
+  passed the build; README "Install" says so, says what `@main` and `v0.0.1`
+  give, and how to check an installed binary. `go.mod` retracts `v0.0.1`, the
+  early build without a database. When the linker set no version,
+  `sfdupes --version` prints the main module's version that Go recorded in the
+  binary, the tag or the pseudo-version of a `go install` build, and `dev` only
+  when neither is known (2026-10-17,
+  https://git.eeqj.de/sneak/sfdupes/issues/116)
+
 - `scan` creates the database's directory where the database path leads: the
   path with its last element cut off as text, never cleaned, so a `..` after a
   symlinked directory is followed through the symlink, as when the file is