docs: document source-of-truth pointers and the CI completion gate

[ChasLui]

What

Two small additions to AGENTS.md, plus a .gitignore entry. Docs/config only — no code changes.

AGENTS.md

• Source of truth — points contributors (and coding agents) at the authoritative references up front: CODING_STANDARDS.md for conventions and docs/architecture.md for the layered request flow / module map.
• Task completion — spells out the exact gate that .github/workflows/rust-checks.yml already enforces, so you can run it locally before pushing:

  cargo fmt --all --check
  cargo check --all-targets --locked
  cargo clippy --all-targets --locked -- -D warnings
  cargo test --locked
  python3 scripts/check_code_size.py   # ≤300 lines/file, ≤50 LOC/function

.gitignore

• Ignore /vendor/ so a local cargo vendor doesn't show up as untracked noise.

Why

New contributors currently have to read the CI workflow to learn what "done" means and where the conventions live. This writes that down where they'll look first. The commands are copied verbatim from the existing workflow, so there's nothing new to maintain — it just mirrors what CI already checks.

Notes

• No source, behavior, or dependency changes.
• Limits (300 lines/file, 50 LOC/function) match scripts/check_code_size.py as of this branch.