Add basic Makefile with help, install, clean, and lint targets

[kluzny]

Add a Makefile to the repo root that provides common development tasks.

Requirements

• make help — default target; prints available targets with descriptions
• make install — installs/symlinks skills locally (wraps ./scripts/install.sh)
• make clean — removes installed symlinks (wraps ./scripts/uninstall.sh)
• make lint — runs shellcheck against all shell scripts in the repo

Notes

• help should be the default target (i.e. running bare make prints help)
• Help output should be auto-generated from ## comments on each target
• lint requires shellcheck to be installed; should fail clearly if it's missing