Document retrieval sidecar rollout

[TheGreenCedar]

Summary

• Adds retrieval sidecar architecture, ops, testing, rollback, and contributor docs, plus updates README/usage/skill references.
• Adds the retrieval sidecar smoke workflow and setup scripts.
• Removes stale internal planning docs superseded by the retrieval docs.

Tests

• node --check scripts/setup-retrieval-env.mjs
• cargo test -p codestory-cli --test onboarding_contracts
• cargo test -p codestory-cli --test architecture_contracts
• cargo fmt --check
• git diff --check

Stack

Base: codex/accuracy-pass-05-benchmark-eval (#15)
Top of stack: this PR. This branch tree matches the original large branch.