library: desktop.ini and Thumbs.db are no more an edit of a skill tha…

[TryWorld2026]

library: desktop.ini and Thumbs.db are no more an edit of a skill than .DS_Store is

What is wrong

A skill given to an agent as a copy (#896) sits in a folder the user opens in a
file manager: Explorer writes desktop.ini and Thumbs.db into it, Finder
.DS_Store. Four walkers of a skill's files each had its own list of what
isn't the skill:

• hashDir left all three in — so the file manager's own bytes changed the skill's hash;
• treeOf and readSkill left .DS_Store out but not desktop.ini or Thumbs.db;
• copyDir wrote whatever the source folder held.

So a copy with Explorer's files in it read as edited: the library took them
into its own folder, every agent was given a copy again, the page said the copy
differed from the library's, and a bundle carried them to another computer with
the skill.

While fixing it: hashDir and readSkill answered .git with return nil
rather than filepath.SkipDir, which kept walking into it — .git's contents
were being hashed as part of the skill.

What changed

SkipInSkill, in skill_edits.go where the list already was, now holds all of
them — magpie's own mark, .git, .DS_Store, desktop.ini and Thumbs.db,
compared without regard to case — and hashDir, treeOf, readSkill and
copyDir all walk by it, so a hash, a tree, a copy and a bundle agree on what a
skill is. The two .git walkers now answer filepath.SkipDir.

Semantic change (Library)

• Before: a skill's hash, tree, copy and bundle each disagreed about what was
  part of the skill, so Explorer's or Finder's own files made a copy read as
  edited and were carried into the library and into bundles.
• After: one list decides it for all four, so a copy with the file manager's
  files in it reads as current and the files are left where they are.
• Reference: docs/subsystems/library.md (updated
  in this PR — the copy's hash contract is where this belongs);
  implementation SkipInSkill in internal/library/skill_edits.go, and its four
  callers in skillcheck.go, skills.go, carry.go.

Verification

Verified on Windows, where Explorer's names are the ones written.

• Without the change TestSkillCopyIgnoresExplorerMetadata fails with
  the file manager's files made the copy look edited: [claude]; its assertions
  run past that one find the library's own folder and codex's copy holding
  desktop.ini, Thumbs.db and THUMBS.DB, and the library's skill kept with
  the backups for the edit.
• TestCarryLeavesExplorerMetadataOut fails with the bundle carries .DS_Store
  and the two Explorer names.
• Both pass with it. go test -tags nogui ./internal/library,
  go vet -tags nogui ./... and the windows, darwin and linux builds pass.

Not in this PR

#916 is a different bug with a different root cause and is untouched here.