feat(command): complete the command line #5

Closes #4

• init scaffolds a project that builds without warnings; --yes accepts defaults.
• template list and template export copy the default layout, partials, and assets.
• pdf publishes the site and renders one PDF per table of contents that sets pdf, with cover and contents pages, header and footer templates, and a configured or detected renderer.
• download and merge handle cross-reference maps.
• --watch rebuilds while serving.
• Published files are written beside their target and renamed into place, keeping the usual file mode.
• Landing pages drop previous and next links; record signatures print without field comments; breadcrumbs follow the deepest matching item.

Validation

pudu check $(find src test tools examples -name '*.pudu')   # no diagnostics
pudu fmt --check src test tools examples                    # clean
pudu lint src test tools examples                           # clean
pudu test test                                              # 13/13 suites, 390 assertions
pudu run examples/BuildSite.pudu                            # builds examples/site