docs: add coverage badge and update task

Amolith created 

Add coverage badge to README and task to update it automatically.

Assisted-by: Claude Opus 4.5 via Crush

Change summary

README.md     |  9 +++++----
Taskfile.yaml | 11 +++++++++++
2 files changed, 16 insertions(+), 4 deletions(-)

Detailed changes

README.md 🔗 

@@ -6,11 +6,12 @@ SPDX-License-Identifier: CC0-1.0
 
 # go-lunatask
 
-[![Go Reference](https://godocs.io/git.secluded.site/go-lunatask?status.svg)][godocs.io]
-[![Go Reference](https://pkg.go.dev/badge/git.secluded.site/go-lunatask.svg)][pkg.go.dev]
+[![Godocs.io Reference](https://godocs.io/git.secluded.site/go-lunatask?status.svg)][godocs.io]
+[![Pkg.go.dev Reference](https://pkg.go.dev/badge/git.secluded.site/go-lunatask.svg)][pkg.go.dev]
 [![Go Report Card](https://goreportcard.com/badge/git.secluded.site/go-lunatask)](https://goreportcard.com/report/git.secluded.site/go-lunatask)
-[![REUSE](https://api.reuse.software/badge/git.secluded.site/go-lunatask)](https://api.reuse.software/info/git.secluded.site/go-lunatask)
-[![Liberapay](https://img.shields.io/liberapay/receives/Amolith.svg?logo=liberapay)](https://liberapay.com/Amolith/)
+![Test coverage](https://img.shields.io/badge/coverage-84.6%25-brightgreen)
+[![REUSE compatibility](https://api.reuse.software/badge/git.secluded.site/go-lunatask)](https://api.reuse.software/info/git.secluded.site/go-lunatask)
+[![Liberapay donation status](https://img.shields.io/liberapay/receives/Amolith.svg?logo=liberapay)](https://liberapay.com/Amolith/)
 
 [godocs.io]: https://godocs.io/git.secluded.site/go-lunatask
 [pkg.go.dev]: https://pkg.go.dev/git.secluded.site/go-lunatask

Taskfile.yaml 🔗 

@@ -48,6 +48,17 @@ tasks:
     cmds:
       - go test -v ./...
 
+  badge:
+    desc: Update coverage badge
+    cmds:
+      - |
+        pct=$(go test -cover ./... 2>&1 | grep -oP 'coverage: \K[0-9.]+')
+        if [ "$pct" = "" ]; then echo "Failed to get coverage"; exit 1; fi
+        color=red; [ "$(echo "$pct >= 50" | bc)" -eq 1 ] && color=yellow
+        [ "$(echo "$pct >= 80" | bc)" -eq 1 ] && color=brightgreen
+        sed -i "s|coverage-[0-9.]*%25-[a-z]*|coverage-${pct}%25-${color}|" README.md
+        echo "Updated badge to ${pct}% (${color})"
+
   vuln:
     cmds:
       - go install golang.org/x/vuln/cmd/govulncheck@latest