Remove Arch-Function, prompt targets, and function calling

[Spherrrical]

Summary

• Remove the Arch-Function runtime path (prompt_gateway → /function_calling → hosted Arch-Function), including the brightstaff handler and CLI auto-inject of the arch-function provider.
• Drop prompt_targets from config/schema/common and simplify prompt_gateway to a passthrough filter.
• Delete related demos, docs, skills, and tests; keep the Envoy plano cluster for Plano-Orchestrator.

Test plan

• cargo test -p common --lib
• cargo test -p brightstaff --lib
• cargo check --target=wasm32-wasip1 -p prompt_gateway -p llm_gateway
• Confirm generated configs no longer include arch-function
• Confirm POST /function_calling returns 404
• Smoke planoai up with a model-listener config (e.g. llm_gateway demo)