perf(project-onboard): derive local path for all repo arg shapes

[joedanz]

Summary

Extends the `$PROJECT_ROOT` local-read optimization to cover the two remaining invocation paths:

• Local path arg (`project-onboard ~/code/my-repo`): `git -C` reads the remote URL and git root from the supplied directory's `.git` folder. No need to also specify the GitHub repo — the skill derives `/` from the remote automatically.
• GitHub URL / owner/name arg (`project-onboard joedanz/Learn-Whatever`): skill now asks "is this cloned locally?" before going remote. If the user provides a path, the local-read path kicks in; if they skip, falls through to the existing `gh api /contents/...` reads.

Phase 2 branching logic (`$PROJECT_ROOT` set → local reads + 1 metadata call; unset → full remote) is unchanged.

Test plan

• `bun test test/skills-conformance.test.ts` passes (134/134)
• `project-onboard ~/code/Learn-Whatever "Learn Whatever" learnwhatever.com` — should set `$PROJECT_ROOT`, derive `joedanz/Learn-Whatever` from `.git`, and use local reads in Phase 2
• `project-onboard https://github.com/joedanz/Learn-Whatever\` — should prompt for local path, then branch accordingly on user response
• `project-onboard` from inside a repo — Case A unchanged, still short-circuits via idempotency gate if already onboarded