docs: define executor streaming boundaries

[franciscojavierarceo]

Summary

• define one target streaming pipeline and assign transport, normalization, ingestion, relay, and orchestration to separate owners
• add mandatory executor-streaming change rules to AGENTS.md, including bounded-memory channels and measurement-gated worker placement
• add a read-only executor-architecture-review skill that checks proposed changes against the repository documentation
• connect the architecture to RFC [RFC]: one linear ingest pipeline for the executor streaming path #241 and the focused follow-ups in refactor: unify synchronous streaming ingestion and item lifecycle #243, refactor: extract StreamRelay and bound client delivery #244, and perf: benchmark inline versus worker placement for streaming ingestion #245 without changing runtime behavior

Test Plan

• quick_validate.py skills/executor-architecture-review
• pre-commit run --files AGENTS.md ARCHITECTURE.md skills/executor-architecture-review/SKILL.md
• cargo fmt --all -- --check
• git diff --check
• cargo test --workspace was attempted, but the machine ran out of disk space while compiling; this PR changes no Rust code