feat: support variables and expansion in config file paths

[utahcon]

Summary

• Add a top-level vars map to the config file; path values can reference $NAME / ${NAME}.
• Variables resolve from vars first, then the environment; they can reference each other. Cycles and undefined variables are reported as errors.
• Leading ~ expands to the home directory and the result is cleaned.
• The config is never rewritten with expanded values (saving keeps the original text), and configHasOrg compares expanded paths.
• configLookupOrg now expands paths and returns an error.
• README updated.

vars:
  WORKSPACE: /my/path/here
  OTHERSPACE: /testing
orgs:
  - org: my-org
    path: $WORKSPACE/$OTHERSPACE   # -> /my/path/here/testing

Test plan

• go build / go vet / gofmt
• Manually verified expansion and the undefined-variable error with a temp config
• Cycle detection and a real sync with a vars-based config

🤖 Generated with Claude Code