Document how to run lintian, piuparts, and autopkgtest independently

[eniko1556]

Context

The lintian, piuparts, and autopkgtest integrations have been removed from pkg-builder's generated Makefiles and sbuild.conf to reduce complexity. These tools are still valuable for package quality, but running them is orthogonal to the build pipeline.

What was removed

• [testing] section in pkg-builder.toml (lintian, piuparts, autopkgtest booleans)
• Tool version tracking for lintian/piuparts/autopkgtest in [tools]
• test-lintian, test-piuparts, test-autopkgtest Make targets
• Lintian opts passed to sbuild via $run_lintian / $lintian_opts in sbuild.conf
• pkg-builder test subcommands

TODO

Document in the user guide how to run these tools independently after a successful build:

# Lintian (static analysis of .deb/.changes)
lintian -i -I --tag-display-limit=0 \
  --suppress-tags bad-distribution-in-changes-file \
  --suppress-tags debug-file-with-no-debug-symbols \
  --fail-on=error --fail-on=warning \
  path/to/*.changes

# Piuparts (install/upgrade/remove testing)
sudo piuparts -d bookworm path/to/*.deb

# Autopkgtest (DEP-8 tests)
sudo autopkgtest path/to/*.changes -- qemu path/to/autopkgtest-bookworm-amd64.img