fix: infer module version for source builds

[joshavant]

Summary

This makes version reporting use Go build metadata as a fallback when release metadata has not been injected by linker flags.

Release builds still work the same way: GoReleaser injects version, commit, and date with -ldflags, and those injected values remain authoritative. For source/module builds without those linker flags, gog now falls back to debug.ReadBuildInfo() and reports the main module version when Go provides one.

Why

go install github.com/steipete/gogcli/cmd/gog@v0.14.0 does not run through GoReleaser, so it does not receive the release workflow's -ldflags. Before this change, those builds fell back to the hardcoded source value (0.13.0-dev), which made a binary built from a newer tag look like an older development build.

That is misleading for diagnostics, support reports, and packaging verification.

Why This Change

This keeps the release workflow and GoReleaser config unchanged while improving source-build behavior:

• GoReleaser artifacts keep their rich injected output, e.g. v0.14.0 (commit date).
• go install module@version can report the module version from Go's build info.
• Plain local development builds still fall back to dev when no useful module version is available.
• Stale hardcoded release-like fallback values are removed.

Verification

• go test ./internal/cmd
• go test ./cmd/gog
• go build -o /tmp/gog-dev ./cmd/gog && /tmp/gog-dev --version prints the local module/VCS-derived build version
• go build -ldflags \"-X github.com/steipete/gogcli/internal/cmd.version=v9.9.9 -X github.com/steipete/gogcli/internal/cmd.commit=abc123 -X github.com/steipete/gogcli/internal/cmd.date=2026-05-01T00:00:00Z\" -o /tmp/gog-release ./cmd/gog && /tmp/gog-release --version prints v9.9.9 (abc123 2026-05-01T00:00:00Z)

Related issue: #544

[steipete]

Thanks for the PR. This fix is already on main as 6af52a4 (fix(version): infer module version for source installs), including the module build-info fallback and regression coverage.

Verified current origin/main has the behavior with go test ./internal/cmd -run TestVersionStringUsesModuleVersionFallback; I also closed #544 with the same proof. Closing this PR as superseded by main, with attribution preserved in the changelog.