Make documented build targets work

[aleksUIX]

Summary

Closes #17. A fresh clone could not follow the documented agent build, and make build-rust failed because rust/cargo.toml is not a Cargo.toml.

This keeps the spec Makefile and the spec README getting-started block. It does not no-op bindings / check / docs. Proto regen remains #11 / PR #18, which already rewires bindings to scripts/generate.sh.

• Add the documented agent targets that were missing (deps, generate -> scripts/generate.sh, docker-build, health-check, run-dev as an alias of run-all, coverage, samples).
• Under Go Reference Implementation: make build uses checked-in pkg/pb/; protoc is not required to build the agent.
• Rename rust/cargo.toml to rust/Cargo.toml so make build-rust works.
• Implement -health-check as a one-shot probe of /health/ready, and point Docker / compose HEALTHCHECK at that binary. The image has no wget/curl. Spec agent-manifest probes stay HTTP GET /health/live and /health/ready.
• Add .dockerignore so docker build does not send rust/target.

Test plan

• make deps && make build && make test && make lint
• make generate invokes scripts/generate.sh (fails without plugins / until Fix proto/generated-code drift breaking gRPC reflection and samples #18; not required to build)
• ./artf-agent -health-check exits 1 with no server, 0 against a running agent
• make health-check curls /health/live and /health/ready
• make build-rust produces rust/target/release/agentic-rtb-framework-service
• docker build then docker run reports Health: healthy via -health-check (no wget)