Update build instructions and Go version

- README: Use make build instead of go build
- Upgrade to Go 1.25

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: nogo

README.md

@@ -159,7 +159,8 @@ go install github.com/nogo/gitree/cmd/gitree@latest
 ```bash
 git clone https://github.com/nogo/gitree.git
 cd gitree
-go build -o gitree ./cmd/gitree
+make build
+# Binary is in bin/gitree
 ```
 
 ## Requirements

go.mod

@@ -1,6 +1,6 @@
 module github.com/nogo/gitree
 
-go 1.24.0
+go 1.25
 
 require (
 	github.com/charmbracelet/bubbles v0.21.0