Restore pkg/errors to go.mod and go.sum — also used by pdfcpu

Incorrectly removed pkg/errors when cleaning up langchaingo deps.
pdfcpu also imports it, so it must be retained.

https://claude.ai/code/session_012X8wf4jwQFRYfMnbbWCEs5

Author: claude

go.mod

@@ -33,6 +33,7 @@ require (
 	github.com/mitchellh/mapstructure v1.5.0 // indirect
 	github.com/ncruces/go-strftime v0.1.9 // indirect
 	github.com/pelletier/go-toml/v2 v2.2.2 // indirect
+	github.com/pkg/errors v0.9.1 // indirect
 	github.com/remyoudompheng/bigfft v0.0.0-20230129092748-24d4a6f8daec // indirect
 	github.com/rivo/uniseg v0.4.7 // indirect
 	github.com/sagikazarmark/locafero v0.4.0 // indirect

go.sum

@@ -60,6 +60,8 @@ github.com/pdfcpu/pdfcpu v0.10.0 h1:K7Hv2tW/poMvG+DuowVGfYvNcN1Y7USS+8IebA3Z8+w=
 github.com/pdfcpu/pdfcpu v0.10.0/go.mod h1:Q2Z3sqdRqHTdIq1mPAUl8nfAoim8p3c1ASOaQ10mCpE=
 github.com/pelletier/go-toml/v2 v2.2.2 h1:aYUidT7k73Pcl9nb2gScu7NSrKCSHIDE89b3+6Wq+LM=
 github.com/pelletier/go-toml/v2 v2.2.2/go.mod h1:1t835xjRzz80PqgE6HHgN2JOsmgYu/h4qDAS4n929Rs=
+github.com/pkg/errors v0.9.1 h1:FEBLx1zS214owpjy7qsBeixbURkuhQAwrK5UwLGTwt4=
+github.com/pkg/errors v0.9.1/go.mod h1:bwawxfHBFNV+L2hUp1rHADufV3IMtnDRdf1r5NINEl0=
 github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
 github.com/pmezard/go-difflib v1.0.1-0.20181226105442-5d4384ee4fb2 h1:Jamvg5psRIccs7FGNTlIRMkT8wgtp5eCXdBlqhYGL6U=
 github.com/pmezard/go-difflib v1.0.1-0.20181226105442-5d4384ee4fb2/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=