feat(structure): allow deep nesting at selected paths

[choplin]

What this PR does

Some valid skill packages intentionally group opt-in asset bundles under paths
such as assets/components/<name>/. Flattening those bundles only to satisfy the
generic deep-nesting warning makes the package harder to maintain.

This PR adds --allow-nested-paths to validate structure and check. The
option suppresses only deep-nesting warnings within explicitly selected,
skill-relative subtrees. Other structure, frontmatter, orphan, token, Markdown,
and link checks continue unchanged.

Paths are normalized across operating systems, matched on path boundaries, and
rejected when absolute or when they escape the skill root. A visible info result
is emitted whenever an exception is applied.

The main implementation lives in:

• structure/paths.go — portable relative-path validation and subtree matching
• structure/checks.go — scoped deep-nesting exception handling
• cmd/{validate_structure,check}.go — matching CLI flags and option propagation
• structure/*_test.go, cmd/exitcode_integration_test.go, and
  testdata/nested-paths-skill/ — boundary, CLI, and output-format coverage

How to test

go test -race ./... -count=1
golangci-lint run

The integration tests cover both commands, comma-separated paths, Windows
separators, invalid absolute/traversal paths, similar-prefix siblings, and text,
JSON, and Markdown output.

Checklist

• Tests pass locally (go test -race ./... -count=1)
• Lint passes locally (golangci-lint run)
• New functionality includes tests
• Breaking changes are noted above (if any)

[dacharyc]

Thank you @choplin — and not just for this PR. Getting three well-scoped, well-tested contributions in a row is a rare treat for this repo, and each one of them reads like it was written by someone who took the time to understand how the codebase is put together. The path-boundary matching (with a fixture specifically covering assets/components vs assets/components-extra), the eager CLI-side validation with clear error messages, and the visible info result when an exception is applied are all exactly how I'd want this feature built.

Reviewed and verified locally: full suite under go test -race ./... -count=1, vet and gofmt clean. Merging as-is once CI confirms.

One heads-up on sequencing: this merges first of your three. #83 shares structure/paths.go with this PR (plus some overlap with the recently-merged #78 in tokens.go), so I'll rebase it myself rather than bounce it back to you — the conflicts are from merge ordering on our side, not anything in your work.