fix: infer Claude install root from setup location

[yasuhirofujii-medley]

Summary

• infer Claude global vs project-local installs from where setup lives instead of from the caller's pwd
• keep explicit --local and --global overrides
• add setup regression coverage for inferred install roots and override handling

Why

Vendored project-local installs currently depend on the caller's working directory. Running setup from inside .claude/skills/gstack can create a nested .claude/skills/gstack/.claude/skills tree instead of installing into the project root.

This PR switches the default to path-based inference so:

• ~/.claude/skills/gstack/setup resolves to global Claude install
• PROJECT/.claude/skills/gstack/setup resolves to project-local Claude install

Notes

• This branch currently includes the runtime-root commit from PR1 because both branches target upstream main.
• Review this after PR1, then it can be rebased down to the install-inference-only delta.

Testing

• bun run gen:skill-docs --host codex
• bun test ./test/gen-skill-docs.test.ts --test-name-pattern 'setup script validation'