fix(installer): create Claude project symlinks

[EfeDurmaz16]

Summary

Fixes project-local Claude Code installs in fresh repositories that do not already have a .claude/ directory.

Previously, project-local symlink installs skipped non-universal agents when the agent root directory was missing. That made sense for avoiding unrelated agent directories, but it also caused Claude Code installs to silently skip .claude/skills/<name> even when Claude Code was the selected target.

Changes

• Allow Claude Code project-local symlink installs to continue when .claude/ does not exist yet.
• Keep the existing skip behavior for other non-universal agents.
• Add a regression test that installs a skill for Claude Code in a fresh project and verifies .claude/skills/<name> is created as a symlink.

Validation

• pnpm format
• pnpm test tests/installer-symlink.test.ts
• pnpm format:check

I also ran pnpm type-check, but it currently fails on existing unrelated errors in untouched files:

• src/git.ts: simple-git options type does not include env
• src/skills.ts: frontmatter metadata is inferred as {} / unknown

Closes #1138.