Add unit-test coverage for the core build engine (worker, store, registry backends)

## Summary

Unit-test coverage is concentrated in a few modules (`start/auth.rs`, `start/registry.rs`, `sdk/rust/src/context.rs` — ~41 test fns across 3 files). The core build orchestration is effectively untested at the unit level and is only exercised indirectly by the end-to-end cross-SDK reproducibility gate in CI.

The e2e digest check is excellent for catching end-state divergence, but it does not pin down unit-level logic bugs, and it makes refactoring the engine high-risk because a regression only surfaces as a whole-build failure.

## Gaps (highest-value first)

- [ ] **Worker** (`cli/src/command/start/worker.rs`, ~1k LOC) — especially the output-rewriting / UTF-8 char-boundary logic around `worker.rs:397-415`, which is exactly the kind of fiddly code that panics on edge cases.
- [ ] **Store** (`cli/src/command/store/*`) — content-addressed hashing, store-path resolution, archive/temp handling.
- [ ] **Registry archive/artifact backends** — local and S3 (`start/registry/archive/{local,s3}.rs`, `start/registry/artifact/{local,s3}.rs`).
- [ ] **Agent** (`cli/src/command/start/agent.rs`) — request handling and lock/source-digest comparison logic.

## Approach

`start/registry.rs` already establishes the right pattern to copy: a `trait ArchiveBackend` / `trait ArtifactBackend` with a `MockBackend` and ~14 table-style tests. Extend that trait-mock approach to the worker/store/backend modules so the engine can be tested without a live daemon or S3.

Suggested follow-ups:
- Surface `cargo test` results as a first-class CI signal (the `build` job already runs `make test`, but coverage isn't tracked).
- Consider a coverage tool (e.g. `cargo llvm-cov`) to set a baseline and prevent regressions.

## Why it matters

This de-risks every future engine refactor and directly supports the panic-removal work in #226 — most of the `unwrap()/expect()` call sites that need converting live in exactly these untested modules.

_Filed as a follow-up to a project review._