feat: make installed binary name overridable via env/make var

install.sh: INSTALLED_NAME defaults to "flashduty" but honours a
caller-supplied INSTALLED_NAME env var so we can install our bundled
copy as "fduty" without shadowing a user's own flashduty installation.
All uses of the installed name (mv target, chmod, PATH message, verify
hint) already reference the variable — only the hardcoded default
literal needed updating.

Makefile: BINARY_NAME switches from := (immediate, non-overridable) to
?= so `make build BINARY_NAME=fduty` produces bin/fduty; default
behaviour (make build) is unchanged.

Public identity (cobra Use: string, help text, FLASHDUTY_* env vars,
Go module, repo name) is untouched.

Author: ysyneu

Makefile

@@ -1,5 +1,5 @@
 # Build configuration
-BINARY_NAME := flashduty
+BINARY_NAME ?= flashduty
 BUILD_DIR := bin
 GOLANGCI_LINT_VERSION := v2.2.1
 GOLANGCI_LINT := $(BUILD_DIR)/golangci-lint

install.sh

@@ -15,7 +15,7 @@ set -e
 
 REPO="flashcatcloud/flashduty-cli"
 BINARY="flashduty-cli"
-INSTALLED_NAME="flashduty"
+INSTALLED_NAME="${INSTALLED_NAME:-flashduty}"
 INSTALL_DIR="${FLASHDUTY_INSTALL_DIR:-/usr/local/bin}"
 
 # When set, all release downloads are fetched from this prefix instead of github.com.
@@ -188,7 +188,7 @@ main() {
            info "  export PATH=\"${INSTALL_DIR}:\$PATH\"" ;;
     esac
 
-    info "Run 'flashduty version' to verify"
+    info "Run '${INSTALLED_NAME} version' to verify"
 }
 
 main