docs: note git as optional dependency for git workspace features

Add prerequisite callout in installation docs and workspace guide.
Add EnsureInstalled runtime check so users get a clear error if
git is not on PATH.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: jahvon

docs/guides/workspaces.md

@@ -27,6 +27,8 @@ When you add a workspace, flow creates a `flow.yaml` configuration file in the r
 
 #### Git Workspaces
 
+> **Prerequisite:** Git must be installed and available on your `PATH` to use git workspace features.
+
 You can also add workspaces directly from Git repositories. Flow clones the repository to its cache directory and registers it as a workspace:
 
 ```shell

docs/installation.md

@@ -5,6 +5,8 @@ title: Installation
 # Installation
 
 > **System Requirements:** flow supports Linux and macOS systems. On Linux, you'll need `xclip` installed to use clipboard features.
+>
+> **Optional:** [Git](https://git-scm.com/) is required for [git workspace](guides/workspaces.md#git-workspaces) features (`workspace add` from URLs, `workspace update`, `sync --git`).
 
 ## Quick Install
 

internal/services/git/git.go

@@ -136,8 +136,20 @@ func ResetPull(repoDir, ref, refType string) error {
 	return runGit(repoDir, "clean", "-fd")
 }
 
+// EnsureInstalled checks that the git binary is available on PATH.
+func EnsureInstalled() error {
+	if _, err := exec.LookPath("git"); err != nil {
+		return fmt.Errorf("git is not installed or not in PATH (required for git workspace features)")
+	}
+	return nil
+}
+
 // runGit executes a git command, streams progress to stderr, and captures output for error messages.
 func runGit(dir string, args ...string) error {
+	if err := EnsureInstalled(); err != nil {
+		return err
+	}
+
 	cmd := exec.Command("git", args...)
 	if dir != "" {
 		cmd.Dir = dir