feat: resolve variable references in source .env before writing to project .env

[castastrophe]

Problem

When a user's ~/.env contains values like:

DB_HOST=localhost
DB_PORT=5432
DATABASE_URL=${DB_HOST}:${DB_PORT}/dev

envoy currently copies ${DB_HOST}:${DB_PORT}/dev literally into the project .env. The resolved value — localhost:5432/dev — is what the project actually needs.

Proposed behavior

Before writing a value to the project .env, envoy resolves any variable references ($VAR or ${VAR}) found in the source value against the other keys present in ~/.env. The resolved string is written, not the template.

Resolution should:

• Follow the same left-to-right, defined-before-use ordering as shell variable expansion
• Detect and warn on circular reference loops rather than hanging
• Leave unresolvable references as-is (same non-destructive fallback behavior as missing keys today)

Open questions

• Should resolution be opt-in via a flag (--resolve-vars) or always-on?
• How should nested references (variables that reference other variables) be handled?
• Should the resolution order be explicitly documented?

Related

Part of the feature roadmap.