fix(build): error on raw go build, require make build

Raw `go build` produces unsigned binaries that macOS kills. Add a
BuiltProperly ldflag that make build sets, and check it at startup.
If missing, print an error directing users to use make build.

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

Makefile

@@ -10,7 +10,8 @@ BUILD_TIME := $(shell date -u +"%Y-%m-%dT%H:%M:%SZ")
 
 LDFLAGS := -X github.com/steveyegge/gastown/internal/cmd.Version=$(VERSION) \
            -X github.com/steveyegge/gastown/internal/cmd.Commit=$(COMMIT) \
-           -X github.com/steveyegge/gastown/internal/cmd.BuildTime=$(BUILD_TIME)
+           -X github.com/steveyegge/gastown/internal/cmd.BuildTime=$(BUILD_TIME) \
+           -X github.com/steveyegge/gastown/internal/cmd.BuiltProperly=1
 
 generate:
 	go generate ./...