docs(phase-5): ship v2 docs to the dev version

[S1M0N38]

Part of #224 — v2 docs rewrite (Diátaxis).

Goal

v2 docs live on the dev version of the docs site; ready for the v2.0.0 tag.

Tasks

• Merge dev-docs → dev; watch deploy_docs.yml deploy to the dev version
• Spot-check coder.github.io/balatrobot/dev/: nav, version selector, llms.txt, search
• On v2.0.0 tag: confirm 2.0.0 version deploy; after main bump, latest flips automatically

Dependencies

• Blocked by: docs(phase-0): audit docs and scaffold the new structure #225, docs(phase-1): rewrite reference pages (api, cli, configuration) #226, docs(phase-2): add explanation pages (architecture, game-state, runtime) #227, docs(phase-3): write how-to guides (install, launch, instances, write-a-bot, contribute) #228, docs(phase-4): add docs-code consistency tests and final polish #229 (all phases)
• Blocks: nothing

Done when

• v2 docs visible on the dev version
• Spot-check checklist verified