feat(git-master): add git_env_prefix config to prefix all git commands

When git-master skill is loaded, all git commands are prefixed with the configured env variable (default: GIT_MASTER=1). This enables custom git hooks to detect git-master skill usage. Set to empty string to disable.

src/config/schema/git-master.ts

@@ -5,6 +5,8 @@ export const GitMasterConfigSchema = z.object({
   commit_footer: z.union([z.boolean(), z.string()]).default(true),
   /** Add "Co-authored-by: Sisyphus" trailer to commit messages (default: true) */
   include_co_authored_by: z.boolean().default(true),
+  /** Environment variable prefix for all git commands (default: "GIT_MASTER=1"). Set to "" to disable. Allows custom git hooks to detect git-master skill usage. */
+  git_env_prefix: z.string().default("GIT_MASTER=1"),
 })
 
 export type GitMasterConfig = z.infer<typeof GitMasterConfigSchema>