refactor: retire runners/lua_runner now that workflow_engine replaces it

[jhamill34]

Closes #73. Executes its own decision record: prototypes/workflow_engine (via #74/#75, now fully landed on main) replaces runners/lua_runner as a whole rather than running alongside it as a second, parallel Lua execution path. Nothing in production depended on the SimpleCode+LUA path (it was only a few days old, never adopted for a real manifest), so this is a straight deletion, not a migration — matching the issue's own reasoning.

What changed

• runners/lua_runner — crate deleted entirely, and its workspace member entry removed.
• binary/apid — its dependency and lua Cargo feature removed; its construction/registration in construct_execution_engine removed; the now-dead LUA_LANG constant removed.
• execution_engine::Engine::run — the Ok(Language::LUA) => ... arm of the SimpleCode dispatch match removed (a comment marks why, referencing Decision: retire runners/lua_runner once workflow_engine's async dispatch path lands #73). The LUA protobuf enum variant itself stays defined, per the issue's own plan — harmless, and a smaller footprint than changing a wire enum.
• binary/apid/tests/lua_e2e.rs and the CI step that ran it (.github/workflows/rust.yml) removed — the only e2e coverage that existed specifically for the retired path.
• runners/workflow_runner's own tests used Language::LUA purely as a convenient "any SimpleCode language" stand-in for its api.run bridge tests (registering a FakeCodeRunner, never the real lua_runner) — switched to Language::JAVASCRIPT so those tests keep exercising Engine::run's real dispatch after the LUA arm is gone. Verified this still genuinely tests dispatch by temporarily registering under a mismatched key and confirming the test fails.

Test plan

• cargo build --workspace and cargo test --workspace clean.
• cargo build -p apid with every feature combination (including workflow) clean.
• cargo clippy --workspace --all-targets clean.
• cargo fmt --all -- --check clean.
• Confirmed no other reference to lua_runner/Language::LUA/the lua feature remains anywhere in the repo outside historical precedent comments (which document design history and stay accurate) and the intentionally-retained LUA proto enum variant.

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