docs: move UsageMiddleware to Observability table, add 0.13.0 compare link

Move UsageMiddleware from Middleware to Observability section in
README API Overview. Add missing [0.13.0] compare link to CHANGELOG.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: tercel

CHANGELOG.md

@@ -499,6 +499,7 @@ Built-in `system.*` modules that allow AI agents to query, monitor
 
 ---
 
+[0.13.0]: https://github.com/aipartnerup/apcore-python/compare/v0.12.0...v0.13.0
 [0.12.0]: https://github.com/aipartnerup/apcore-python/compare/v0.11.0...v0.12.0
 [0.11.0]: https://github.com/aipartnerup/apcore-python/compare/v0.10.0...v0.11.0
 [0.10.0]: https://github.com/aipartnerup/apcore-python/compare/v0.9.0...v0.10.0

README.md

@@ -63,7 +63,6 @@ A schema-enforced module standard for the AI-Perceivable era.
 | `ErrorHistoryMiddleware` | Records errors into ErrorHistory |
 | `PlatformNotifyMiddleware` | Emits events on error rate/latency spikes |
 | `ObsLoggingMiddleware` | Observability-aware structured logging middleware |
-| `UsageMiddleware` | Per-call usage tracking middleware |
 
 **Schema**
 
@@ -83,6 +82,7 @@ A schema-enforced module standard for the AI-Perceivable era.
 | `ContextLogger` | Context-aware structured logging |
 | `ErrorHistory` | Ring buffer of recent errors with deduplication |
 | `UsageCollector` | Per-module usage statistics and trends |
+| `UsageMiddleware` | Per-call usage tracking middleware |
 | `TraceContext` | W3C Trace Context propagation (traceparent/tracestate) |
 | `InMemoryExporter` | Span exporter that stores spans in memory |
 | `StdoutExporter` | Span exporter that writes spans to stdout |