fix(skills): unbreak main after fork-skill refactor merge

[dgageot]

Why

main is currently broken: package github.com/docker/docker-agent/pkg/runtime fails to build because pkg/runtime/skill_runner.go references identifiers that no longer exist in scope:

pkg/runtime/skill_runner.go:67:5:  undefined: skill
pkg/runtime/skill_runner.go:68:45: undefined: skill
pkg/runtime/skill_runner.go:73:14: undefined: params
pkg/runtime/skill_runner.go:74:14: undefined: skill

The breakage was introduced by the merge of two PRs that touched handleRunSkill:

• Skills: allow fork skills to override the model #2525 — Skills: allow fork skills to override the model — added a model-override block using local variables skill.Model and params.Name.
• refactor(skills): move fork-skill validation into SkillsToolset #2524 — refactor(skills): move fork-skill validation into SkillsToolset — refactored handleRunSkill to delegate to SkillsToolset.PrepareForkSubSession, dropping the local skill variable and renaming params → args.

The merge applied #2524 cleanly on top of #2525 but did not update the new model-override block, leaving four references to undefined identifiers.

Fix

Restore the build by surfacing the model override through the toolset, in line with the refactor's intent of keeping skill-specific business rules out of the runtime.

• Add Model string to PreparedSkillFork and populate it from skill.Model inside (*SkillsToolset).PrepareForkSubSession. The runtime no longer needs a direct skills.Skill reference.
• Switch handleRunSkill's override block to prepared.Model and prepared.SkillName. The WithAgentModel call, defer restore(), and warning log are otherwise unchanged — runtime behaviour is identical to what Skills: allow fork skills to override the model #2525 intended.
• Extend TestSkillsToolset_PrepareForkSubSession to assert the Model field is propagated, and add TestSkillsToolset_PrepareForkSubSession_NoModelOverride to pin down the empty-Model contract the runtime relies on to skip the override path.

Validation

• go build ./... — clean
• go vet ./... — clean
• golangci-lint run ./pkg/runtime/... ./pkg/tools/builtin/... — 0 issues
• go test ./pkg/runtime/... ./pkg/tools/builtin/... ./pkg/skills/... ./pkg/agent/... — all green

Diffstat

 pkg/runtime/skill_runner.go      |  8 ++++----
 pkg/tools/builtin/skills.go      |  4 ++++
 pkg/tools/builtin/skills_test.go | 19 ++++++++++++++++++-
 3 files changed, 26 insertions(+), 5 deletions(-)

Assisted-By: docker-agent