feat: add plan manager and budgeting layer

[chaddevmachine]

deep-research-align:A – Planning & Budget Layer

Current	Proposed (Deep Research)
Planner returns string list only; no artifact.	PlanManager keeps structured steps + budgets with trace metadata.
Costs tracked post-hoc; no budgets or stops.	Configurable token/cost/web budgets with optional enforcement.
No plan trace or token accounting.	Plan trace persists to JSON/logs; tokens recorded with costs.
Summary: PlanManager tracks steps and budgets and can emit plan traces without altering CLI/API behaviour.
Verify: pip install pytest; pytest tests/test_plan_manager.py

[chaddevmachine]

Branch merged.