Five fixes against @pdmack's review of the contributor docs rewrite
(#1143 (review)):

- **Fix broken anchor** in maintaining.md → recipe.md#bom-regeneration
  (the heading is `## BOM Regeneration`, no `-rule` suffix).
- **Document readiness phase** in validator.md. The three
  container-per-validator phases (deployment, performance,
  conformance) were already clear, but `readiness` is a first-class
  ValidationConfig field that uses inline constraint evaluation in
  `checkReadiness`, not a phase container. Added a note so
  contributors don't go looking for it under the phase containers.
- **Add `pkg/recipe/oskind`** to the index.md package table.
  Imported by pkg/recipe, pkg/collector, pkg/snapshotter, and the
  CLI to break a cycle that previously forced OS string constants
  to be duplicated.
- **Clarify merge precedence arrow** in recipe.md. Annotated the
  arrow with `(lowest priority)` / `(highest priority)` markers and
  added a one-line note that the arrow is priority, not temporal
  order — the narrative already said this, but the diagram
  benefits from the inline hint.
- **Drop fragile line-range links** in api-server.md
  (`middleware.go#L45-L61`, `errors.go#L127`). Both reference
  source files that get edited frequently and the line range goes
  stale silently. File-level links are sufficient; the file is
  small enough that readers can navigate.

`make lint` green.