Add a requirements_approach setting to settings.json

[ivklgn]

Problem

All three requirement tracks exist, but a project can't declare which approach it prefers, so tooling can't tailor its guidance.

Expected result

An optional requirements_approach setting (e.g. product | sources | iso) in settings.json, following the existing optional-settings convention (omitempty + code-level default).

Impact

Lets downstream tooling (track hints, doctor checks) adapt to the project's chosen methodology instead of guessing.

Scope

• Add the optional field to the settings schema.
• Validate allowed values; reject unknown ones.
• Cover load/validate/marshal round-trips with tests.

Files: internal/config/config.go (+ tests)

Acceptance: the field parses, validates, and round-trips; omitting it preserves current behavior.

Only useful alongside a consumer (e.g. the doctor advisory or the cascade hints) — coordinate accordingly.