pre-compute brew shellenv at install time for faster shell startup

Instead of running `eval "$(brew shellenv)"` on every shell start
(~100-200ms subprocess), generate the output once during cps init
and save to ~/shell/env/brew.zsh. rc-base.zsh sources the static
file instead.

Author: Tanq16

internal/configs/rc-base.zsh

@@ -1,12 +1,6 @@
 # --- Homebrew ---
 export HOMEBREW_NO_AUTO_UPDATE=1
-if [ -x /opt/homebrew/bin/brew ]; then
-  eval "$(/opt/homebrew/bin/brew shellenv)"
-elif [ -x /home/linuxbrew/.linuxbrew/bin/brew ]; then
-  eval "$(/home/linuxbrew/.linuxbrew/bin/brew shellenv)"
-elif [ -x /usr/local/bin/brew ]; then
-  eval "$(/usr/local/bin/brew shellenv)"
-fi
+[ -f "$HOME/shell/env/brew.zsh" ] && source "$HOME/shell/env/brew.zsh"
 
 # --- Oh My Zsh ---
 export ZSH="$HOME/.oh-my-zsh"

internal/runner/runner.go

@@ -49,6 +49,7 @@ func Init(ghToken string) {
 		p.ShellExecDir(),
 		filepath.Join(p.ShellDir(), "rc"),
 		filepath.Join(p.ShellDir(), "rc", "custom"),
+		filepath.Join(p.ShellDir(), "env"),
 		filepath.Join(p.ShellDir(), "custom"),
 		filepath.Join(p.ConfigDir(), "extensions"),
 	} {
@@ -239,6 +240,7 @@ func runPostInstall(p platform.Platform) {
 		}
 	}
 
+	generateShellEnv(p, &errors, &lineCount)
 	generateCompletions(p, &errors, &lineCount)
 
 	utils.ClearLines(lineCount + 1) // sub-lines + running header
@@ -252,6 +254,31 @@ func runPostInstall(p platform.Platform) {
 	}
 }
 
+func generateShellEnv(p platform.Platform, errors *[]jobResult, lineCount *int) {
+	envDir := filepath.Join(p.ShellDir(), "env")
+	if err := os.MkdirAll(envDir, 0755); err != nil {
+		*errors = append(*errors, jobResult{name: "shell-env", err: err})
+		return
+	}
+
+	brewBin, err := exec.LookPath("brew")
+	if err != nil {
+		return // brew not found, nothing to generate
+	}
+
+	utils.PrintIndentedRunning("shell-env: brew")
+	*lineCount++
+	cmd := exec.Command(brewBin, "shellenv")
+	out, err := cmd.Output()
+	if err != nil {
+		*errors = append(*errors, jobResult{name: "shell-env-brew", err: err})
+		return
+	}
+	if err := os.WriteFile(filepath.Join(envDir, "brew.zsh"), out, 0644); err != nil {
+		*errors = append(*errors, jobResult{name: "shell-env-brew", err: err})
+	}
+}
+
 func generateCompletions(p platform.Platform, errors *[]jobResult, lineCount *int) {
 	compDir := filepath.Join(p.ShellDir(), "completions")
 	if err := os.MkdirAll(compDir, 0755); err != nil {