Drop unit tests the acceptance suite already covers

[jon-devlapaz]

Summary

Acceptance tests are the behavior sensors. In-module unit tests that only repeated those rows, or that restated formatting and happy-path copies, are removed (about 1,600 lines).

Unit tests that still catch a bug acceptance does not assert stay in place: rollback and orphan retention, digest framing and file mode, symlink and special-file boundaries, and updater parser, digest, and archive checks.

AGENTS.md already states the testing rules for future agents:

• Never write unit tests after you write code.
• Highly prefer E2E tests as the sole testing mechanism. Use them to verify complex features work. At the end of E2E tests, produce a verifiable and repeatable artifact.
• If you must test a system in isolation, first write down all the ways it could fail, then write the code.

docs/TESTING.md now points at the sensors that remain.

Test plan

• cargo test --lib --locked --offline — 50 remaining unit tests passed
• cargo test --test acceptance_traceability --test workflow_contract --locked --offline — passed
• cargo fmt --all -- --check and cargo clippy --workspace --all-targets --locked --offline -- -D warnings — passed
• cargo test --tests --locked --offline --no-run — acceptance binary compiled