docs: update ARCHITECTURE.md and README.md for v1.2.0

- ARCHITECTURE.md: Added natural language date resolution section,
  hide_zero_spend parameter docs, updated example output, changelog
- README.md: Added changelog section with v1.2.0 and v1.1.0 entries
- Updated version metadata and creation date

README.md

@@ -56,9 +56,22 @@ Both valves are passed at runtime — no hardcoded keys. The tool is portable to
 
 ## Documentation
 
-- **[ARCHITECTURE.md](ARCHITECTURE.md)** — Full architecture and design doc: API endpoints, response processing, stripped fields, error paths, future considerations.
+- **[ARCHITECTURE.md](ARCHITECTURE.md)** — Full architecture and design doc: API endpoints, response processing, stripped fields, error paths, future considerations, changelog.
 - **This README** — Quick reference: install, functions, valves, design principles.
 
+## Changelog
+
+### v1.2.0 (2026-06-13)
+- **feat:** Natural language date shortcuts for `litellm_cost()` — `today`, `yesterday`, `monday`, `last week`, etc.
+- **feat:** `hide_zero_spend` parameter for `litellm_cost()` — filters sub-$0.001 models from breakdown
+- **fix:** Cost report header shows resolved YYYY-MM-DD dates instead of raw shortcuts
+
+### v1.1.0 (2026-06-13)
+- Initial public release
+- Three functions: `litellm_health`, `litellm_cost`, `litellm_history`
+- Valve-based configuration (no hardcoded keys)
+- Token-efficient response formatting
+
 ## License
 
 MIT