Remove the Astra model aliases and follow upstream #342

Remove J5's Astra model aliases and follow upstream

J5 maps astra, gpt-6 and 6 to gpt-6-astra in packages/contracts/src/model.ts (FORK.md case 32). This was added on 2026-09-04 (PR #101) to mirror the Fable aliases that existed then. Upstream has since removed its Claude aliases, and it now has gpt-6-astra as its default model with no astra alias. Jackson (2026-09-28): follow upstream.

Change:

• Remove the three aliases and case 32.
• Check D4's Astra exception (apps/server/src/j5/a2a/DeliveryTransport.ts, which compares the normalized model to gpt-6-astra). It must still match threads that were started with the full model id. Anything that stored astra as a model name needs a look.
• Remove the register entry "Astra model aliases" from docs/j5/product/upstream.md.