feat: lazy validation, only resolve expressions for the active context

[MattDevy]

Summary

• Restructure loadConfig to use two-stage validation: structural first, then full validation only on the active context
• Add StructuralConfigSchema that validates the outer config shape (current_context, contexts keys, commands) without deeply validating context values
• Resolve expressions only for the active context and commands section (in parallel via Promise.all)
• Inactive contexts with unresolvable expressions (e.g. unset env vars) no longer cause failures

Closes #144

Test plan

• Active context resolves while inactive context has unset env var (core bug fix)
• Active context with unresolvable expression still fails
• --use-context override resolves only the selected context
• Structural validation catches: missing current_context, empty contexts, nonexistent context ref
• Commands section still resolves and validates
• Invalid URL in active context rejected after expression resolution
• All 145 tests pass (133 existing + 8 new + 4 store)
• Clean tsc and eslint

[margaretjgu]

lgtm! since LoadConfig now contains the same lines from resolveContext, i think we can either refactor resolveContext so loadConfig can still call it (instead of duplicating), or add a comment saying this function is no longer used by loadConfig, kept for external callers...for future maintenance if we need ot update the context in two places.

not urgent just nitpick

[MattDevy]

Whoops sorry, got trigger happy on merging!

I'll raise a PR to fix this!