feat: natural language date shortcuts for litellm_cost()

Adds _resolve_date() helper that converts natural language shortcuts
into YYYY-MM-DD before calling the LiteLLM API.

Supported shortcuts:
- today, yesterday, tomorrow
- N days ago (e.g. "two days ago")
- monday-sunday (most recent occurrence)
- last monday-last sunday
- this week, last week
- this month, last month

Falls back to passthrough for already-formatted YYYY-MM-DD strings.

Updated README with new parameter documentation.

README.md

@@ -11,8 +11,8 @@ Check LiteLLM proxy health and authentication status. Returns which models are h
 Get LiteLLM cost breakdown for a date range. Returns per-model and per-day spend, token counts, cache hit rate, and request stats.
 
 **Parameters:**
-- `start_date` (str, required): Start date in YYYY-MM-DD format
-- `end_date` (str, required): End date in YYYY-MM-DD format
+- `start_date` (str, required): Start date — YYYY-MM-DD or natural language shortcut (e.g. "2026-06-01", "today", "yesterday", "monday", "last monday", "two days ago", "this week", "last month")
+- `end_date` (str, required): End date — YYYY-MM-DD or natural language shortcut
 - `hide_zero_spend` (bool, default false): Hide models with sub-$0.001 spend. Useful when free-tier or subscription models (e.g. Ollama Cloud) dominate the breakdown with high token counts but negligible cost.
 
 ### `litellm_history`