Make a local dev tool tryable: state dir, PATH symlink, reversible install

When handing a work-in-progress local tool to its user to try, symlink the release binary into a directory already on their PATH rather than asking them to type a target/release path, keep mutable state out of the repo (a state dir under the users home works and survives rebuilds), and state the exact uninstall steps in the same breath as the install — especially when the install appended lines to a shell rc file. Append shell-rc lines with a marker comment naming what added them so removal is unambiguous later. Worth noting a freshly initialised git repo with zero commits makes git rev-parse --abbrev-ref HEAD return the literal string HEAD, which leaks into any branch-aware feature during early development and looks like a bug.