Remove top-level engineering directory

[jcwal1516]

Summary

• remove the generic top-level engineering/ documentation drawer
• keep the live 0.8.1 public-API evidence under docs/release-evidence/public-api/
• consolidate durable repository-policy guidance into CONTRIBUTING.md
• update CI planning, semver checks, release documentation, and repository-lint coverage for the new ownership
• link historical 0.7.3 and 0.8.0 evidence to their versioned tags instead of carrying generated reports on main

Why

The old directory mixed current release evidence with completed audits, generated historical reports, and drafts. This keeps the repository layout focused and closer to Burn's approach: durable contributor guidance lives with contributor documentation, current release evidence has a narrowly named home, and historical artifacts live in Git history and release tags.

Verification

• python3 -m unittest scripts.tests.test_ci_plan — 12 passed
• cargo test -p xtask --test repo_lint — 48 passed on the current working tree
• cargo test -p xtask semver::tests — 29 passed
• cargo xtask fmt — passed
• cargo clippy -p xtask --all-targets --all-features -- -D warnings — passed
• git diff --check — passed

Known base issue

When this commit is replayed in isolation against committed main, cargo test -p xtask --test repo_lint release_facing_docs_match_the_latest_release_tag fails because README.md on main still identifies 0.8.0 while the latest tag is v0.8.1. That mismatch predates this PR. Unrelated, unstaged release-reconciliation work was intentionally excluded from this commit.