Add build script, AGENTS.md, fix formatting

- scripts/build: gofmt, go vet, go build, docker build
- AGENTS.md: agents must run ./scripts/build before committing
- Fix gofmt issues in handler/auth.go and store/store.go

AGENTS.md

@@ -0,0 +1,23 @@
+# Agent Instructions
+
+## Before Committing
+
+Always run the build script before committing any code changes:
+
+```sh
+./scripts/build
+```
+
+This runs:
+- `gofmt` check (unformatted code is a hard failure)
+- `go vet`
+- `go build`
+- `docker build`
+
+Fix every error and warning before proceeding to commit. If `gofmt` reports
+unformatted files, run `gofmt -w .` and re-check.
+
+## Commit Messages
+
+Use concise, descriptive commit messages in imperative mood. Keep the first line
+under 72 characters.

handler/auth.go

@@ -136,6 +136,7 @@ func (h *AuthHandler) ServeSignout(w http.ResponseWriter, r *http.Request) {
 	})
 	http.Redirect(w, r, "/signin", http.StatusSeeOther)
 }
+
 type contextKey string
 
 const userIDKey contextKey = "userID"

scripts/build

@@ -0,0 +1,29 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+cd "$(dirname "$0")/.."
+
+echo "==> gofmt check"
+unformatted=$(gofmt -l .)
+if [ -n "$unformatted" ]; then
+    echo "Unformatted files:"
+    echo "$unformatted"
+    gofmt -d .
+    echo "Run: gofmt -w ."
+    exit 1
+fi
+echo "    ok"
+
+echo "==> go vet"
+go vet ./...
+echo "    ok"
+
+echo "==> go build"
+CGO_ENABLED=0 go build -o podstalk .
+echo "    ok"
+
+echo "==> docker build"
+docker build -t podstalk .
+echo "    ok"
+
+echo "==> all checks passed"