chore: update code generation and styleguide instructions concerning use of auto keyword

.agents/AGENTS.md

@@ -44,3 +44,42 @@
       MetricType(HasSubstr("outstanding_rpcs")),
       HasMetricLabel("channel_pool_lb_policy", Eq("RANDOM_TWO_LEAST_USED")))));
   ```
+
+## Type Deduction and `auto` Guidelines (Abseil Tip #232)
+
+Follow [Abseil Tip of the Week #232](https://abseil.io/tips/232) and the Google
+C++ Style Guide: use `auto` only if it makes code clearer or safer, not merely
+to avoid typing an explicit type.
+
+- **Range-Based `for` Loops Over Maps and Associative Containers:**
+  - Use `auto` with structured bindings (`for (auto const& [key, value] : map)`)
+    when iterating over `std::map`, `absl::flat_hash_map`, protobuf maps (e.g.,
+    `metadata()`, `labels()`), and JSON `.items()`.
+  - *Why:* Iterating with `std::pair<Key, Value>` triggers implicit conversions
+    and unintentional deep copies because map elements are
+    `std::pair<const Key, Value>`. Structured bindings eliminate this hazard and
+    remove `kv.first` / `kv.second` noise.
+- **Standard Factory Functions:**
+  - Use `auto` when the type is explicitly specified on the RHS with standard
+    factory functions (e.g., `auto client = std::make_shared<MockClient>();`,
+    `auto ptr = std::make_unique<T>(...);`).
+- **Iterators:**
+  - Use `auto` for iterator variables when the container type is clearly
+    declared in the local scope (`auto it = local_vec.begin();`).
+  - When the container is not local (e.g., a class member variable), either
+    spell out the iterator type or explicitly bind the dereferenced element type
+    (e.g., `ElementType const& elem = *it;`).
+- **Spell Out Domain Types, Protobufs, and Return Values:**
+  - Do not use `auto` where it obscures domain types, protobuf messages, or
+    function return types (e.g., avoid `auto actual = client.InsertObject(...)`;
+    use `StatusOr<ObjectMetadata> actual = ...`).
+  - Do not use `auto` for nested protobuf access (e.g., avoid
+    `auto const& field = proto.nested().field()`; spell out the protobuf/string
+    type).
+- **Avoid `auto` for Primitive / Numeric Types:**
+  - Use explicit types (`std::size_t`, `std::int64_t`, `std::uint32_t`, etc.)
+    instead of bare `auto` initialized with integer literals.
+- **Explicit Semantics (`const`, `&`, `*`):**
+  - When a reference or pointer is intended, always explicitly qualify as
+    `auto const&`, `auto&`, or `auto*` to prevent accidental copies (since bare
+    `auto` deduces by value) and to make ownership and mutability unambiguous.

.gemini/styleguide.md

@@ -72,6 +72,34 @@ When reviewing or generating code, apply rigorous scrutiny:
 - **Documentation:** Document all public items in libraries with Doxygen style
   comments using `///`.
 
+## Type Deduction (`auto`)
+
+Apply the type deduction rules from
+[Abseil Tip #232](https://abseil.io/tips/232) and the Google C++ Style Guide
+during code reviews:
+
+- **Require Structured Bindings in Map Loops:** In range-based `for` loops over
+  maps, protobuf maps, and JSON `.items()`, require `auto` with structured
+  bindings (e.g., `for (auto const& [key, value] : map)`). Flag loops that use
+  `std::pair` or trigger implicit copy conversions.
+- **Factory Functions:** Allow `auto` when the type is explicitly written on the
+  RHS (e.g., `auto ptr = std::make_unique<T>(...);`,
+  `auto client = std::make_shared<T>(...);`).
+- **Iterators:** Allow `auto` for iterators only when the container type is
+  locally visible. When iterating over non-local/member containers, require
+  explicitly typed dereferences (e.g., `ElementType const& elem = *it;`).
+- **Reject Obscured Domain & Return Types:** Flag and reject `auto` when it
+  hides `StatusOr<T>`, domain objects, protobuf messages/fields, or function
+  return types (e.g., demand
+  `StatusOr<ObjectMetadata> actual = client.InsertObject(...)` instead of
+  `auto actual = ...`).
+- **Disallow `auto` for Primitives:** Require explicit numeric and scalar types
+  (`std::size_t`, `std::int64_t`, `bool`, etc.) rather than deducing them from
+  literals.
+- **Enforce Explicit Qualifiers:** Ensure `auto` is explicitly qualified with
+  `const`, reference (`&`), or pointer (`*`) (e.g., `auto const&`, `auto&`,
+  `auto*`) to prevent unintended copies or ambiguous mutability.
+
 ## Google Cloud SDK Specifics
 
 - **Generated Code:** Do not edit files with a "Generated by the Codegen C++