feat(pi): make Astra the default model (#293) ## Summary - enable GPT-6 Astra and make it the default Pi model - update subagent routing with Astra as the main coordinator and hard-task escalation model - remove unused Fable and pi-claude-bridge configuration - document official Astra capability, cost, and token-efficiency evidence ## Checks - pre-commit hooks passed - nixfmt, deadnix, statix, and ripsecrets passed - independent reviewer passed - no full Home Manager switch or full build was run Co-authored-by: Ammar Nanjiani <ammar.nanjiani@gmail.com> Reviewed-on: https://codeberg.org/ananjiani/infra/pulls/293