Make script/lint fail on any Hugo warning (closes #25)

The lint step in Dockerfile.lint now passes --panicOnWarning alongside
--printPathWarnings. Until now Hugo printed "Duplicate target paths"
when two pages wrote the same output file and still exited 0, so the
lint passed while one page was silently lost. The flag turns that
warning, and every other Hugo warning, into a failed lint.

script/test and the deploy build are unchanged. The comments in
Dockerfile.lint, script/lint and script/check, the README entry for
script/lint, and TODO.md now describe the stricter lint.

Model: opus-5-5

README.md

@@ -66,9 +66,10 @@ provide:
   git pre-commit hook
 - `script/test` — the correctness check: a clean `hugo --minify` production
   build
-- `script/lint` — a clean build that surfaces broken links and path collisions,
-  run inside Docker: it builds `Dockerfile.lint`, where the lint is a build
-  step, so a successful build is a clean lint
+- `script/lint` — a Hugo build that fails on any Hugo warning, including two
+  pages that write the same output path, run inside Docker: it builds
+  `Dockerfile.lint`, where the lint is a build step, so a successful build is a
+  clean lint
 - `script/fmt` — format every markdown and CSS file in the repo with prettier;
   the exclusions live in `.prettierignore` with the reason for each
 - `script/fmt-check` — check that formatting (read-only)