Add helper scripts for use, sync, and freshness

[thegalexc]

Summary

This adds small helper scripts for the three most mechanical library operations:

• bin/use.py
• bin/sync.py
• bin/freshness.py

It also updates the skill docs, cookbooks, and justfile so the documented workflow matches the actual behavior.

The goal is to keep the agent as the orchestration layer while moving repetitive file-copy, clone, and compare logic into simple reusable scripts.

Why

The current library concept is strong, but some operations are very procedural:

• resolve an entry from library.yaml
• fetch from local or GitHub source
• install dependencies first
• compare installed content to source
• refresh installed items consistently

Encoding those steps entirely in prose makes behavior harder to keep consistent across runs and across agent harnesses.

These helpers make the core operations:

• repeatable
• testable
• easier to document
• easier for agents to invoke correctly

What’s included

New scripts

• bin/use.py

  • install or refresh one catalog entry
  • supports local paths and GitHub URLs
  • installs dependencies first
  • supports --global
  • supports --target
  • supports --project-root for resolving relative default_dirs

• bin/sync.py

  • re-pulls all installed items
  • processes dependencies before dependents
  • supports --dry-run
  • supports --project-root

• bin/freshness.py

  • compares installed content against source via live hashing
  • supports local and GitHub-backed sources
  • supports --json
  • supports --project-root

Safer project-local path handling

Relative default_dirs are now resolved against a target project root, not implicitly against whatever directory the script was launched from.

That prevents a subtle footgun where running a command from the library repo itself could accidentally install or inspect project-local entries there.

Docs and command surface updates

• adds /library freshness to the documented command set
• adds cookbook/freshness.md
• updates use, sync, and related cookbooks
• adds a just freshness shortcut
• updates README wording so it no longer claims there are no helper scripts

Behavior notes

• local file and local directory sources are both supported
• GitHub browser and raw URLs are supported
• freshness checks all installed copies it finds
• the scripts support older Python via typing imports rather than newer-only annotation syntax

Validation

Manually validated with a temp project:

1. install a demo skill into a project-local default dir
2. confirm freshness reports up-to-date
3. modify installed content
4. confirm freshness reports outdated
5. run sync
6. confirm freshness returns to up-to-date

Also ran:

• python3 -m py_compile bin/use.py bin/sync.py bin/freshness.py
• git diff --check

Scope

This PR intentionally avoids any private catalog entries, private skills, or repo-specific customizations. It is limited to generic helper-script functionality and the docs needed to support it.

[no-tably]

Very useful!