fix(coding-agent): preserved fork prompt cache affinity

- Persisted an inherited provider prompt-cache key on full session forks while keeping the child OMP session id independent.

- Added --prompt-cache-key and SDK startup inheritance so explicit cache affinity is separate from provider session routing.

- Cleared automatic inherited keys when model, thinking, system prompt, or tool schema inputs change.

Fixes #5035

packages/coding-agent/src/cli/args.ts

@@ -42,6 +42,7 @@ export interface Args {
 	noSession?: boolean;
 	sessionDir?: string;
 	providerSessionId?: string;
+	providerPromptCacheKey?: string;
 	fork?: string;
 	/** Collab link to join at startup (set by the `join` subcommand; no CLI flag). */
 	join?: string;