docs: align env and troubleshooting docs with the code (#12404)

Four documentation claims contradicted the code: .env.example called OMNIROUTE_USE_TURBOPACK dev-only and said the production build still uses webpack (it reads the same flag and defaults to Turbopack); the README's Bun section said `bun run build` auto-detects Bun and switches to Webpack (only `bun run dev` does — the production bundler is decided by the flag alone); TROUBLESHOOTING.md gave OMNIROUTE_CHAT_MAX_HEAVY_IN_FLIGHT a default of 1 when unset means no request-count cap; and it quoted the pre-#12223 wording of the structural 503 chat_admission_busy message. The Retry-After bullet in the same section is deliberately untouched because #12395 rewrites it.

Validated in a combined worktree with all 25 PRs of this batch boarded together: typecheck:core clean, 443/443 node-runner tests plus 14/14 vitest across every test file the batch touches, and check-changelog-integrity, check:cycles (418 files), check:provider-consistency (272 REGISTRY entries, 355 canonical providers), check:docs-counts, check:docs-sync (42 locales) and check-file-size all green.

Thanks @pacocartones.