chore(toolchain): keep .tool-versions -> .mise.toml pin conversion (R-16)

Owner ruling 2026-08-28 (R-16/R-20/R-21): keep the pin conversion from the
template-sync sweep, revert the rest. Pin content verified against
HEAD:.tool-versions before commit.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: hyperpolymath

.mise.toml

@@ -0,0 +1,11 @@
+[tools]
+# = "Uncomment and customize for your project"
+# = "rust nightly"
+# = "just 1.40.0"
+# = "nickel 1.10.0"
+# = "gleam 1.8.0"
+# = "elixir 1.18.0"
+# = "erlang 27.2"
+# = "zig 0.14.0"
+# = "idris2 0.7.0"
+rust = "nightly"