Upgrade Bun to 1.4.2

Update the pinned Bun version in package metadata, mise.toml, the
Docker image, CI, the agentic workflow, and documentation. Bump the
@types/bun catalog entry to 1.4.2.

Assistant-model: Claude Opus 5.5
Assistant-workflow: inline
Assistant-duration: 5m converged, estimated 5m
Assistant-verification: bun run ci passed: dprint, oxlint, token checks under Bun 1.4.2
Assistant-verification: bun run types passed: all workspace type checks under Bun 1.4.2
Assistant-verification: bun test passed: 1523 pass, 2 skip, 0 fail under Bun 1.4.2
User-preference: Keep the mise.toml Bun pin in step with Bun upgrades
Co-authored-by: Alex Lavaee <lavaman131@github.com>