feat: Support linking workspace-level `.tank` skills to agents

[elad12390]

Problem

When you install skills locally (tank install @org/skill without --global), they land in ./.tank/skills/ within the project. The install flow does link these to agents using the local .tank/agent-skills/ directory, but there's no standalone command to re-link or manage these workspace-level links after the fact.

tank link today only operates on the current directory as a skill source (for skill authors doing local dev). There's no way to say "take everything in this project's .tank/skills/ and link it to my agents" — which is what you'd want as a skill consumer working in a project.

Expected behavior

A command (or flag on tank link) that:

1. Reads the local ./.tank/skills/ directory (or skills.lock)
2. Links each installed skill to detected agent skill directories (Claude, Cursor, OpenCode, etc.)
3. Targets the agent's workspace-scoped paths, not global ~/.tank/

Key files

• apps/cli/src/commands/link.ts — current link command (skill-author focused)
• apps/cli/src/commands/install.ts — already does local linking during install (line ~274: .tank/agent-skills/)
• apps/cli/src/lib/linker.ts — symlink creation logic