agents: add Rust positional arg count and local execution rules (#4412)

Fixed-arity Rust CLI parsers used match guards and fixed-size array
`try_from` conversions for positional argument counts, and per-mnemonic
`--strategy=Rustc=local` overrides caused host `glibc` version
mismatches on remote test runners.

Require explicit length checks for fixed positional argument counts and
`--spawn_strategy=local` when overriding remote execution locally.

.agents/rules/rust.md

diff --git a/.agents/rules/rust.md b/.agents/rules/rust.md
index e105a6c..939086b 100644
--- a/.agents/rules/rust.md
+++ b/.agents/rules/rust.md
@@ -46,6 +46,9 @@
   (`bool`, enums, validated `PathBuf`, pre-compiled `Regex`) in `parse_args`
   rather than storing stringly-typed `String` fields or deferring validation to
   execution.
+* **Positional argument counts**: Validate fixed positional argument counts with
+  an explicit length check (e.g., `if positional.len() != 3`) rather than
+  collection-length match guards or fixed-size array `try_from` conversions.
 * **`OsString` & `ExitCode`**: Keep CLI arguments and file paths as `OsString`
   and `PathBuf`; never round-trip `OsString -> String -> PathBuf` via
   `to_string_lossy()`. Return `std::process::ExitCode` from `main()` and

.agents/rules/testing.md

diff --git a/.agents/rules/testing.md b/.agents/rules/testing.md
index a7fdba2..c141507 100644
--- a/.agents/rules/testing.md
+++ b/.agents/rules/testing.md
@@ -35,6 +35,12 @@
 * When building `//docs:docs` fails with exit code 2, treat it as a known
   Sphinx/Bazel flake and retry the build.
 
+## Local vs. Remote Execution Consistency
+* Do not pass `--strategy=Rustc=local` (or per-mnemonic local overrides for
+  compiled binaries) while leaving test runners or downstream build actions on
+  remote execution; host `glibc` mismatches cause `GLIBC_2.xx not found` on RBE
+  containers. Use `--spawn_strategy=local` when running locally.
+
 ## Test Organization
 * **Domain Placement**: Place tests by the feature tested (e.g.,
   `tests/bootstrap_impls/`), not by the fixture helper used (e.g.,