chore(linux): own ignore, editorconfig and generated-file attributes

[datlechin]

Why

Linux build dirs were ignored partly by the root file and partly not at all, editors used the root's 4-space indent on Meson, GtkBuilder and CSS files, and the banned-words check exempted license text by a hard-coded Linux path list.

Change

• linux/.gitignore owns _build/, adds _flatpak/ and .flatpak-builder/, and drops lines the root covers plus docs/audit-*.md, a local file nothing references.
• linux/.editorconfig overrides the root with the indentation the files already use.
• .gitattributes marks the Linux generated files linguist-generated, and verbatim license texts and the SQLite headers linguist-vendored.
• check-banned-words.sh skips a diff file with either attribute set.
• Two packaging scripts get the shellcheck directives their notes asked for.

Verified

The new test_check_banned_words.py fails on the old script. Old and new flag the same lines over seven bases and a staged change, minus the newly vendored files; shellcheck 0.9.0 is clean.