chore: remove legacy STATELESS_VALIDATOR_LOG_* env var migration

[flyq]

Summary

• Drop the migrate_legacy_env_vars() shim in stateless-common::logging that copied STATELESS_VALIDATOR_LOG_* into the newer STATELESS_LOG_* names at startup.
• Remove the corresponding call in bin/stateless-validator/src/app.rs::run() and its import.
• Update README.md so the Logging Configuration section documents only the canonical STATELESS_LOG_FILE_DIRECTORY / STATELESS_LOG_FILE / STATELESS_LOG_STDOUT variables.

The legacy names duplicated the new ones and were only kept as a transitional alias; with the rename settled, the migration helper is no longer needed.

Breaking change

Users still setting STATELESS_VALIDATOR_LOG_FILE_DIRECTORY, STATELESS_VALIDATOR_LOG_FILE, or STATELESS_VALIDATOR_LOG_STDOUT must rename them to the STATELESS_LOG_* equivalents. Command-line flags (--log-*) are unchanged.

Test plan

• cargo build / cargo check --workspace
• cargo fmt --all --check and cargo clippy --workspace --all-targets --all-features
• Run stateless-validator with STATELESS_LOG_FILE_DIRECTORY=./validator-data/logs STATELESS_LOG_STDOUT=info and confirm file + stdout logging behave as before.
• Run with the old STATELESS_VALIDATOR_LOG_* names and confirm they are now ignored (no silent fallthrough).

[claude]

The PR currently has no labels, but it looks like at least the documentation label is missing — the diff includes updates to README.md (the Logging Configuration section was revised to reflect the canonical STATELESS_LOG_* variable names).

There is no "breaking-change" label available in this repo, but it may be worth noting that the PR description itself flags this as a breaking change (users must rename STATELESS_VALIDATOR_LOG_* env vars). If such a label exists or is created, it would apply here too.

[claude]

LGTM. Clean removal of the migration shim, README updated correctly, breaking change clearly documented. No issues.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.9%. Comparing base (686f3ba) to head (a02f752).
⚠️ Report is 1 commits behind head on main.

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[claude]

LGTM. Clean removal — no dangling references remain, the README is updated, and the breaking change is clearly called out in the description.