feat: add playbook-library end-user CLI

[KyleAMathews]

Summary

• Adds a new playbook-library bin to @tanstack/playbooks — a lean end-user CLI separate from the existing maintainer CLI
• Libraries (e.g. @tanstack/router) wire it up by running playbook setup --shim, which generates a bin/playbook.js shim they commit and expose as their own playbook bin entry
• When a consumer runs playbook list, the CLI walks up from the shim's path to detect the home package, then recursively traverses dependencies + peerDependencies looking for other packages with bin.playbook, collecting and printing skills from each
• playbook install prints an agent-driven prompt (like scaffold) that maps skills to project tasks and writes a <!-- playbook-skills:start/end --> block into CLAUDE.md

New files

• src/library-scanner.ts — recursive dep traversal + skill discovery, testable via optional projectRoot param
• src/playbook-library.ts — list and install commands
• tests/library-scanner.test.ts — 9 tests covering skill data, recursive traversal, peerDeps, cycle detection, sub-skills, and error cases

Changes

• src/setup.ts — new --shim flag (on by default) generates bin/playbook.js shim with instructions
• package.json — playbook-library bin entry, ./playbook-library subpath export, updated build script

Test plan

• pnpm build completes cleanly
• pnpm test:lib — all 127 tests pass
• node dist/playbook-library.mjs list prints home package + skills
• node dist/playbook-library.mjs install prints agent prompt and copies to clipboard
• playbook setup --shim generates bin/playbook.js with correct content

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commit: f3cc850