fix: consolidate env-var resolver in api/client.ts with unified ENV_VAR_PATTERN

Found during v0.5.21 release review. `api/client.ts:59` has a separate env-var resolver (`\$\{env:([^}]+)\}`) that diverges from the unified `ENV_VAR_PATTERN` in `config/paths.ts`. This creates a third resolution grammar that makes it hard to reason about what gets resolved where.

Deferred because: consolidation refactor across multiple call sites (>30 min).