chore: rename mise config and add tasks

[lox]

Summary

This switches the repo over to a standard mise.toml config and adds a couple of project tasks for the common local workflows.

The main change is that mise run test and mise run lint now work out of the box. The test task uses go tool gotestsum, which matches how CI already runs tests, instead of installing a separate gotestsum binary through mise. That keeps tool versioning in one place, via the Go tool block in go.mod.

Changes

• rename .mise.toml to mise.toml
• add a test task for running go tool gotestsum
• add a lint task for running go tool gofumpt -extra -w . and golangci-lint run
• remove the separate mise-managed gotestsum dependency and use the repo-pinned Go tool instead

Why

This makes the local developer workflow simpler without introducing another source of truth for tool versions.

In particular:

• gotestsum now follows the same pattern locally and in CI
• gofumpt and gotestsum are both sourced from the repo's Go tool definitions
• mise is used for the top-level workflow commands, not for duplicating tool version management

Testing

• mise tasks ls
• mise run test -- -run '^$' ./...
• mise run lint currently reports existing repo lint failures unrelated to this change