GeezerVison

When ReadSkillOptions.ResolveAlias is nil, read_skill and read_skill_file build a workspace skill from GetSkills and serve it. Only tests leave the resolver nil. The one production caller, skillOpts in turn_environment.go, always sets it to skillspkg.Lookup over the merged personal, organization, project, and workspace skills.

This PR deletes resolveSkillAlias and that fallback. Without a resolver, both tools now return skill alias resolver is not configured, as they already do for the other unset options. The tests resolve names with the real skillspkg.MergeSkills and skillspkg.Lookup instead of hand-written fakes. read_skill_file still serves a workspace skill hidden from the model (#31147): it checks for a nil resolver before resolveSkillFileTarget, which now calls ResolveAlias directly. The PR adds 89 lines and removes 160.

Why it is safe

• git grep 'ReadSkillOptions{' finds one non-test caller, and it sets ResolveAlias. Behavior with a resolver does not change.
• GetSkills and findSkill stay. They still supply the pinned body, the directory, and the supporting files of a resolved workspace skill.
• TestReadSkillTool/ModelHiddenSkill is deleted. It checked that the fallback hid a skill that disables model invocation. With the real resolver, TestModelHiddenWorkspaceSkillsLeftOutOfModelSkills checks that, and it fails when the model-invocable filter in workspaceSkillsForResolution is removed. ListedSkillServesFileThatDisablesModelInvocation now sets a resolver.

Mutants of skill.go (go test -overlay, -run TestReadSkill), on main at 27db7ac and on this branch:

A copy of skill.go with a syntax error fails to build in both runs, so the overlay applies.

Review focus

• The qualified alias tests are stronger. The fakes returned the alias they got, so on main a tool that echoed resolved.Alias passed. The real lookup resolves personal/my-skill to the bare alias my-skill, so the tests now check that the tool returns the requested name.
• CollisionAliasRoundTrip no longer calls the resolver itself. It checks each response body instead. The fake said that a bare deploy was not found; the real lookup says it is ambiguous, which AmbiguousLookupSurfacesAliases covers.
• ResolveAliasErrorIsSanitized keeps a fake, because the real lookup returns only not-found and ambiguous errors.
• feat: serve plugin skills in chatd #29323 (plugin skills) replaces ResolveAlias and rewrites resolveSkillAlias, so it conflicts with this PR. Whichever merges second must reconcile.

Validation

• go test ./coderd/x/chatd/chattool/ -count=1: pass.
• go test ./coderd/x/chatd/chattool/ -race -count=3 -run Skill: pass.
• go test ./coderd/x/chatd/ -run Skill: pass.
• go vet ./coderd/x/chatd/chattool/ ./coderd/x/chatd/: no issues.
• Mutants: each fails as listed.
• make pre-commit with CODER_HOOK_RUN_ALL=1 passed.

Xum, an AI agent, opened this PR for @ibetitsmike and wrote this description.