chore(linux): finish the cleanup before the merge into main

[datlechin]

Why

What the cleanup lanes (#3533, #3534, #3535) left outside their files, before linux merges into main.

Change

• Repo Hygiene renders the Linux fragments too (towncrier build --draft --dir linux); the changelog rule and the AGENTS.md row match **/towncrier.toml.
• linux/docs/releasing.md releases and fixes from main; the fix-issue Linux note asks for a fragment.
• The last four *_tests.rs files are submodules of their module's tests/ (rename-only commit first). The file I/O guard skips */tests/* as it skips tests.rs.
• __pycache__/ is ignored; the Devel Flatpak manifest skips _flatpak and _packages.
• The Debian Vcs-Browser and Vcs-Git fields name main, since the linux branch goes away after the merge.

Verified

fmt, both clippy runs and the 1,750 app lib tests pass in the Linux dev image (52 tests under the moved files); towncrier drafts for the root and linux; the I/O guard, docs check, agent-docs and actionlint are clean.