Document missing rustfmt validation tool

docs/report/2026-08-19-coder-toolchain-missing-rustfmt.md

@@ -0,0 +1,17 @@
+# Ticket validation image lacks `rustfmt`
+
+While implementing T-613, `cargo fmt --all` failed before reading project sources because the active Rust toolchain does not provide the `rustfmt` component. The standalone `rustfmt` binary is also absent.
+
+Observed commands:
+
+```text
+$ cargo fmt --all
+error: 'cargo-fmt' is not installed for the toolchain '1.90.0-x86_64-unknown-linux-gnu'.
+
+$ rustfmt --version
+bash: rustfmt: command not found
+```
+
+This prevents Workers from executing the repository-required formatting check or automatically normalizing Rust changes. The Coder can still run compile/tests and `git diff --check`, but formatting evidence remains weaker and review may surface avoidable formatting differences.
+
+Suggested improvement: include the `rustfmt` component in the standard coding Workdir/toolchain image and verify `cargo fmt --all -- --check` during image qualification.