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

Owner ruling 2026-08-28 (R-16/R-28c): keep the pin conversion from the
template-sync sweep, revert the rest. Committed locally on the current branch;
this repo's HEAD tracks no upstream (R-28c lane) so no merge/push here.

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

Author: hyperpolymath

.mise.toml

@@ -0,0 +1,4 @@
+[tools]
+rust = "stable"
+deno = "2.2.8"
+deno = "2.2.8"