Lazy validation: only resolve expressions for the active context

[MattDevy]

Summary

Expression resolution currently runs eagerly across all contexts in the config, not just the active one. This means if an inactive context references an unset environment variable, the CLI will fail even though that context isn't being used.

Example

current_context: local
contexts:
  local:
    elasticsearch:
      auth:
        api_key: $(env:LOCAL_KEY)
  staging:
    elasticsearch:
      auth:
        api_key: $(env:STAGING_KEY)

Running with current_context: local will fail if STAGING_KEY is unset, because all expressions are resolved before Zod validation.

Proposed Solution

Split validation into two stages:

1. Structural validation of the outer config shape (all contexts, but without resolving expressions)
2. Full validation of only the active context after expression resolution

This requires restructuring the validation pipeline so that Zod doesn't reject unresolved expressions (like $(env:X)) as invalid values during the first pass.

Context

Discussion from #143 between @MattDevy and @JoshMock.