Report the Go build info version when linker flags set none (closes #422)

A `go install` of a tag sets no linker flags, so the binary reported
`dev`. Version, commit and commit date that the linker flags leave
unset now come from the build info Go records. Go records a
pseudo-version for an untagged commit and a `+dirty` suffix for a
modified checkout; `IsDevVersion` now counts both as development
builds, using `golang.org/x/mod`.

Model: opus-5-5

README.md

@@ -1041,20 +1041,37 @@ is `git describe --tags --always --dirty`, which `script/version` runs for the
 A `docker build .` of a clone, with no build arguments, runs the same
 `git describe` (without `--dirty`) on the `.git` in its build context, so it
 stamps the same value for a clean commit; the build fails if the context carries
-`.git` and no version, commit or commit date comes out. A binary built without
-git metadata reports `dev`, or `unknown` when `script/docker` or
-`script/cibuild` built it outside a git checkout.
+`.git` and no version, commit or commit date comes out. `make` outside a git
+checkout and `docker build .` without `.git` both stamp `dev`. `script/docker`
+and `script/cibuild` stamp `unknown` when they run outside a git checkout.
+
+A binary built by plain `go install` or `go build` carries no stamped version.
+It reports the version Go recorded in it instead:
+
+- a `go install` of the tag `v1.0.0` → `v1.0.0`.
+- a `go build` of an untagged commit → a Go pseudo-version, such as
+  `v1.0.1-0.20261110231226-0123456789ab`.
+- a build where Go recorded no version → `dev`.
 
 `goreleaser` stamps a release binary with the tag minus its leading `v`, so the
 tag `v1.0.0` produces `vaultik 1.0.0`, matching the archive name
 `vaultik_1.0.0_linux_amd64.tar.gz`. `goreleaser --snapshot` stamps
 `dev-<12 chars of the commit sha>` rather than inventing the next patch number.
-`vaultik version` calls a build a development build when its version is `dev`,
-`unknown`, `dev-<sha>`, the short commit sha or `<tag>-<N>-g<short sha>`, with
-or without `-dirty`; only a plain tag, such as `v1.0.0` or `1.0.0`, is a
-release. If `script/version` cannot be run at all, `make` stops with an error
-instead of building an unversioned binary, and a binary that somehow carries an
-empty version string still reports itself as a development build.
+
+`vaultik version` calls a build a development build when its version is one of
+these:
+
+- `dev` or `unknown`.
+- `dev-<sha>`.
+- the short commit sha.
+- `<tag>-<N>-g<short sha>`.
+- a Go pseudo-version.
+- any version with a `-dirty` or `+dirty` suffix.
+
+Only a plain tag, such as `v1.0.0` or `1.0.0`, is a release. If `script/version`
+cannot be run at all, `make` stops with an error instead of building an
+unversioned binary. A binary stamped with an empty version reports the version
+Go recorded in it; if Go recorded none, it is a development build.
 
 ### cutting a release