feat: hint the resources layout for narrowed exports

[jwmoss]

Export hint

export prints one hint line after it writes the export:

Hint: this export selects one or more resource groups. Use --module-layout resources to call the individual Registry modules. See docs/module-layouts.md.

Conditions for the hint:

• The user passed --resources or --name.
• The effective --module-layout is workspace.
• The export wrote its files. A partial export that blocks Terraform output prints no hint.

The hint uses the existing rc.progress() stderr channel, so --quiet and --json suppress it.
needsLayoutHint holds the condition.

Documentation

docs/resource-selection.md gains a "Selection and module layout" section. It states that
--resources and --name choose the objects, that --module-layout chooses the modules and the
Terraform addresses, and that a narrower selection does not change the layout. It includes the
--resources catalogs --module-layout resources --scaffold example and links to
docs/module-layouts.md.

Changelog

CHANGELOG.md is new. It uses the Keep a Changelog format and covers v0.1.0 through v0.5.0 from
the merged pull requests in each tag range, plus an Unreleased section for this change.
CONTRIBUTING.md gains one sentence that asks contributors to add an Unreleased entry with each
pull request.

Tests

TestExportHintsResourceLayout in internal/cli/export_test.go is table-driven. It covers
--resources, --name, the default groups, --module-layout resources, and --quiet. The two
positive cases failed before the change.

make check passes.

Intended for release v0.5.1. This pull request creates no tag and no release.