chore: Add Makefile, smoke script, and act config for local dev

[imthaghost]

Summary

• Makefile collects the everyday rituals: build / test / fmt / clippy / deny, plus end-to-end smoke and act-driven local CI mirroring
• scripts/smoke.sh runs the full local stack (mailbox + transit + CLI send/receive) and verifies the file round-trips byte-perfect — useful regression check before pushing
• .actrc pins a sane default runner image (catthehacker/ubuntu:medium-latest) and disables the Windows/macOS matrix slots that can't run locally

Stacked on chore/fix-ci so CI passes on this branch. Rebase onto main when #2 lands.

Test plan

• make help — lists targets
• make test — green (uses the same network-test skips as CI)
• make fmt-check — clean
• make smoke — full roundtrip succeeded locally; receiver hash matches sender hash
• make ci, make ci-fmt etc. require Docker; not exercised here

Notes

• Tool prerequisites are listed at the top of the Makefile (rustup nightly, cargo-deny, act, Docker).
• RUSTUP_CARGO defaults to $HOME/.cargo/bin/cargo so fmt/fmt-check work even when Homebrew rust is first on \$PATH.
• The CI-mirroring targets (make ci-*) need the Docker daemon running; this is documented at the top of the Makefile.