Normalize package spec dependency names in four README files

[Copilot]

The spec audit reported 4 cross-package inconsistencies in Dependencies naming conventions despite full spec coverage and no stale docs. This PR standardizes those entries to the canonical internal import-path format used across package specs.

• Scope

  • Updates only package specifications (pkg/*/README.md), no runtime code changes.
  • Touches 4 packages: errorutil, sliceutil, stats, syncutil.

• Dependency naming normalization

  • Replaces shorthand internal references (pkg/logger) with canonical fully-qualified paths (github.com/github/gh-aw/pkg/logger) in each affected ## Dependencies section.
  • Aligns these docs with existing conventions used in other package READMEs.

• Representative change

  - `pkg/logger` — package-scoped logging used by `OnceLoader[T]`.
  + `github.com/github/gh-aw/pkg/logger` — package-scoped logging used by `OnceLoader[T]`.