test: cover Qoder environment variable parsing

[heimanba]

What changed

• add a representative agents.yaml fixture with Qoder Agent environment variables
• verify the public YAML loader preserves the environment_variables map

Why

The Qoder environment-variable support from PR #77 depends on this field surviving YAML schema parsing. A direct regression test prevents future schema changes from silently stripping the values before planning and apply.

Impact

Test-only change. Runtime behavior is unchanged.

Validation

• bun test packages/sdk/tests/unit/parser.test.ts
• bun run verify:scoped
• pre-push bun scripts/verify.ts push (full typecheck, architecture checks, tests, and lint)