chore: modernize CI and update Go toolchain

- Bump Go from 1.19 to 1.26 and update all dependencies
- Rewrite CI workflow with matrix strategy (Linux, macOS, Windows)
- Update GitHub Actions to current versions (checkout@v4, setup-go@v5)
- Update CodeQL actions from v1 to v3
- Fix cross-platform bug in mock/path.go (path.Join -> filepath.Join)
- Clean up dependabot config (weekly schedule, remove stale ignore)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

go.mod

@@ -1,38 +1,37 @@
 module github.com/cheat/cheat
 
-go 1.19
+go 1.26
 
 require (
-	github.com/alecthomas/chroma/v2 v2.12.0
+	github.com/alecthomas/chroma/v2 v2.23.1
 	github.com/davecgh/go-spew v1.1.1
 	github.com/docopt/docopt-go v0.0.0-20180111231733-ee0de3bc6815
-	github.com/go-git/go-git/v5 v5.11.0
+	github.com/go-git/go-git/v5 v5.16.5
 	github.com/mattn/go-isatty v0.0.20
 	github.com/mitchellh/go-homedir v1.1.0
 	gopkg.in/yaml.v3 v3.0.1
 )
 
 require (
-	dario.cat/mergo v1.0.0 // indirect
-	github.com/Microsoft/go-winio v0.6.1 // indirect
-	github.com/ProtonMail/go-crypto v0.0.0-20230923063757-afb1ddc0824c // indirect
-	github.com/cloudflare/circl v1.3.7 // indirect
-	github.com/cyphar/filepath-securejoin v0.2.4 // indirect
-	github.com/dlclark/regexp2 v1.10.0 // indirect
+	dario.cat/mergo v1.0.2 // indirect
+	github.com/Microsoft/go-winio v0.6.2 // indirect
+	github.com/ProtonMail/go-crypto v1.3.0 // indirect
+	github.com/cloudflare/circl v1.6.3 // indirect
+	github.com/cyphar/filepath-securejoin v0.6.1 // indirect
+	github.com/dlclark/regexp2 v1.11.5 // indirect
 	github.com/emirpasic/gods v1.18.1 // indirect
 	github.com/go-git/gcfg v1.5.1-0.20230307220236-3a3c6141e376 // indirect
-	github.com/go-git/go-billy/v5 v5.5.0 // indirect
-	github.com/golang/groupcache v0.0.0-20210331224755-41bb18bfe9da // indirect
+	github.com/go-git/go-billy/v5 v5.7.0 // indirect
+	github.com/golang/groupcache v0.0.0-20241129210726-2c02b8208cf8 // indirect
 	github.com/jbenet/go-context v0.0.0-20150711004518-d14ea06fba99 // indirect
-	github.com/kevinburke/ssh_config v1.2.0 // indirect
-	github.com/pjbgf/sha1cd v0.3.0 // indirect
-	github.com/sergi/go-diff v1.3.1 // indirect
-	github.com/skeema/knownhosts v1.2.1 // indirect
+	github.com/kevinburke/ssh_config v1.5.0 // indirect
+	github.com/klauspost/cpuid/v2 v2.3.0 // indirect
+	github.com/pjbgf/sha1cd v0.5.0 // indirect
+	github.com/sergi/go-diff v1.4.0 // indirect
+	github.com/skeema/knownhosts v1.3.2 // indirect
 	github.com/xanzy/ssh-agent v0.3.3 // indirect
-	golang.org/x/crypto v0.17.0 // indirect
-	golang.org/x/mod v0.14.0 // indirect
-	golang.org/x/net v0.19.0 // indirect
-	golang.org/x/sys v0.15.0 // indirect
-	golang.org/x/tools v0.16.1 // indirect
+	golang.org/x/crypto v0.48.0 // indirect
+	golang.org/x/net v0.50.0 // indirect
+	golang.org/x/sys v0.41.0 // indirect
 	gopkg.in/warnings.v0 v0.1.2 // indirect
 )