feat(ralph-loop): add strategy option for fresh context per iteration

Closes #1901

Add 'default_strategy' config option (default: 'continue') to control whether ralph-loop creates a new session per iteration ('reset') or keeps the same session ('continue'). The 'reset' strategy keeps the model in the smart zone by starting with fresh context for each iteration.

Supports --strategy flag for per-command override.

src/config/schema/ralph-loop.ts

@@ -7,6 +7,7 @@ export const RalphLoopConfigSchema = z.object({
   default_max_iterations: z.number().min(1).max(1000).default(100),
   /** Custom state file directory relative to project root (default: .opencode/) */
   state_dir: z.string().optional(),
+  default_strategy: z.enum(["reset", "continue"]).default("continue"),
 })
 
 export type RalphLoopConfig = z.infer<typeof RalphLoopConfigSchema>