[Codinit] Implement planning and multi-step agent functionality

Summary Add a planning layer and multi-step agent capability so agents can decompose tasks into ordered subtasks, maintain context across steps, and reliably execute multi-action workflows. Motivation Current single-turn agents are limited for complex tasks that require planning, intermediate decisions, or external actions (APIs, I/O). Enabling multi-step agents will: - Allow complex workflows (e.g., research → draft → verify → publish). - Improve reliability by making execution explicit and recoverable. - Make automation and orchestration use-cases practical. Goals - Provide a planner that decomposes high-level goals into discrete steps/actions. - Provide a step executor that runs actions sequentially, maintains context, and handles failures/retries. - Expose a clear API for building multi-step agents and plugging in tool actions (APIs, shell commands, database ops). - Persist context and execution history so agents can resume and audit runs. Non-goals - Building domain-specific pl