Pin Bun 1.3.2 with mise

Add mise.toml so contributors using mise get the repository-pinned Bun
automatically, and list it among the places to keep synchronized.

Assistant-model: Claude Opus 5.5
Assistant-workflow: inline
Assistant-duration: unmeasured converged, estimated 5m
Assistant-verification: mise exec passed: `mise exec -- bun --version` reports 1.3.2 in the checkout
Assistant-verification: dev supervisor passed: `mise exec -- bun run dev` passes the Bun version gate and serves /api/session 200
Assistant-verification: dprint passed: `bunx dprint check AGENTS.md mise.toml`
User-preference: Manage the Bun version with a committed mise.toml, kept in its own PR separate from feature work
Co-authored-by: Alex Lavaee <lavaman131@github.com>