chore: add mise config for dev environment setup

[vmaerten]

Summary

• Add a mise.toml at the repo root so contributors can install every tool needed to work on Task with a single mise install.
• Pin the versions already used across the project: Go 1.25.10 (go.mod), Node 24, pnpm 11.5.0 (website/package.json), golangci-lint 2.12.2 (CI), mockery 3.2.2 (Taskfile.yml), plus gotestsum, goreleaser (v2) and gorelease.
• pnpm uses the npm: backend (the aqua package has broken macOS assets for 11.5.0).
• Purely additive: the Taskfile.yml, CI workflows and go.mod are untouched; the existing install:* tasks keep working alongside mise.
• Document mise install as the recommended setup in contributing.md, keeping the manual instructions as a fallback.

Test plan

• mise install installs all tools; mise ls shows the pinned versions resolved from mise.toml.
• Verified effective versions: go 1.25.10, node 24, pnpm 11.5.0, golangci-lint 2.12.2, mockery v3.2.2, gotestsum v1.13.0, goreleaser 2.16.0, gorelease.
• End-to-end smoke test: mise exec -- go build ./cmd/task builds successfully.