Ignore relative XDG_* values and reject a relative HOME, per the XDG
spec; add a hint to the missing-config error; tighten docs and comments.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
An installed pkgwatch has no checkout to run from, so packages.d/,
state/ and work/ can no longer be relative to the working directory.
New paths module resolves them per the XDG base-directory spec, with
PKGWATCH_{CONFIG,STATE,WORK}_DIR overrides for dry runs.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>