Add missing errorutil package spec and align dependency sections in package READMEs

[Copilot]

The spec audit reported a missing package specification for pkg/errorutil and dependency-list drift in pkg/cli and pkg/parser specs. This PR closes those documentation gaps so package specs reflect current imports.

• Add missing package specification

  • Introduced pkg/errorutil/README.md with:
    • package overview
    • public API table (IsNotFoundError)
    • usage example
    • dependencies and design notes
    • standard spec-extractor footer

• Align dependency declarations with actual usage

  • Updated pkg/cli/README.md dependencies to include:
    • github.com/github/gh-aw/pkg/errorutil
  • Updated pkg/parser/README.md dependencies to include:
    • github.com/github/gh-aw/pkg/errorutil
  • repoutil and stringutil remain explicitly listed in pkg/cli/README.md as expected by the audit.

• Spec/API example added for errorutil

  import "github.com/github/gh-aw/pkg/errorutil"
  
  if errorutil.IsNotFoundError(err) {
      // Handle missing resource path
  }

[Copilot]

Pull request overview

Adds a missing package specification README for pkg/errorutil and aligns the dependency lists in the pkg/cli and pkg/parser package READMEs to reflect their actual imports of errorutil.

Changes:

• Add new pkg/errorutil/README.md documenting the IsNotFoundError helper, usage, and design notes.
• Add pkg/errorutil to the dependency lists in pkg/cli/README.md and pkg/parser/README.md.

Show a summary per file

File	Description
pkg/errorutil/README.md	New package spec describing the IsNotFoundError helper and its design.
pkg/cli/README.md	Adds errorutil to the internal dependencies list.
pkg/parser/README.md	Adds errorutil to the internal dependencies list.

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• Files reviewed: 3/3 changed files
• Comments generated: 0