feat(xerrors): initial stable release v0.9.0

Structured application errors with typed codes, cause chaining, key-value context fields, and zero-import logz enrichment bridge.

What's included:
- `*Err` type implementing error, errors.Unwrap, json.Marshaler, ErrorCode(), and ErrorContext()
- Twelve typed Code constants aligned with gRPC canonical status names
- New / Wrap factory constructors plus InvalidInput / NotFound / Internal convenience constructors
- Builder methods WithContext and WithError for attaching structured fields and causes
- Duck-typed ErrorCode() / ErrorContext() bridge so logz auto-enriches log records without an import

Tested-via: todo-api POC integration
Reviewed-against: docs/adr/

.gitignore

@@ -0,0 +1,38 @@
+# Binaries
+*.exe
+*.exe~
+*.dll
+*.so
+*.dylib
+
+# Test binary, built with go test -c
+*.test
+
+# Output of go build
+*.out
+
+# Dependency directory
+vendor/
+
+# Go workspace file
+go.work
+go.work.sum
+
+# Environment files
+.env
+.env.*
+
+# Editor / IDE
+.idea/
+.vscode/
+*.swp
+*.swo
+*~
+
+# OS
+.DS_Store
+Thumbs.db
+
+# VCS files
+COMMIT.md
+RELEASE.md