Default the hosted agent model to gpt-6-luna

claude-sonnet-4.6 is deprecated and Copilot now rejects session creation
with "Model is not available". Move the server default, the example
environment, and the self-hosting reference to gpt-6-luna.

Assistant-model: Claude Opus 5.5
Assistant-workflow: inline
Assistant-duration: unmeasured converged, estimated 10m
Assistant-verification: agent-browser manual scenario passed: @chopin with MODEL=claude-sonnet-4.6 failed session.create; with MODEL=gpt-6-luna the Planner replied "Hello!" on a local document
Assistant-verification: typecheck passed: bun run types
Assistant-verification: bun test unit passed: bun test apps/server/src (704 pass, 2 skip, 0 fail)
Assistant-verification: dprint and oxlint passed: changed files
User-preference: Stack follow-up feature PRs on the mise branch (alex/mise-bun-pin) until it merges
Co-authored-by: Alex Lavaee <lavaman131@github.com>