Adds a `mise.toml` pinning Bun 1.3.2, the version already required by `package.json`, CI, the Docker image, and `scripts/dev.ts`. Contributors who use [mise](https://mise.jdx.dev) get the right Bun inside the checkout after `mise install`, while their global Bun stays untouched. Contributors without mise are unaffected.

`AGENTS.md` now mentions `mise.toml` and lists it among the places to update when the Bun version changes.

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>