cli: rename LIGATE_LGT_TOKEN_ID env var to LIGATE_AVOW_TOKEN_ID (post-#42 follow-up)

[proofmancer]

Summary

#42 renamed every LGT-prefixed identifier in this repo to AVOW (prose, base units, internal Rust idents, wire-deserialize fields) but missed the clap env = attribute on --token-id. This PR closes that gap.

Concrete: in src/transfer.rs and src/balance.rs, #[arg(long, env = "LIGATE_LGT_TOKEN_ID")] → env = "LIGATE_AVOW_TOKEN_ID". README updated to match.

Breaking surface

Breaking on env-var name only. Operators who set export LIGATE_LGT_TOKEN_ID=token_1... in their shell rc need to rename the variable. No flag-name change, no wire-format change. The CLI's --token-id flag is unaffected.

CHANGELOG.md historical [0.2.2] section is intentionally NOT rewritten, LIGATE_LGT_TOKEN_ID was what shipped with v0.2.2; history stays accurate. New [Unreleased] entry documents the rename.

Test plan

• Local cargo check + cargo fmt --check pass (pre-commit hook ran).
• CI green on this PR.
• Next cli release (v0.2.3 candidate) carries the rename + the [Unreleased] entry folds into the new version section.