ci(linux): make main the home of the Linux app

[datlechin]

Why

The last step before linux merges into main. The Linux workflow skipped itself on main through a detect job and a weekly dispatcher, filtered by paths: so it could never be a required check, and package builds and release tags expected the linux branch.

Change

• build-linux.yml follows the macOS pattern: a Changes (Linux) job runs a new linux suite of detect-changed-paths.sh, every job is skipped by if: instead of paths:, and one Linux job fails when any job failed or was cancelled. Linux is the check main's ruleset can require.
• Package builds run on pushes to main; a linux-v* tag must be on LINUX_BRANCH, now main when unset.
• The Linux guides, the fix-issue Linux platform and the root README, AGENTS, CONTRIBUTING, RELEASING and CI rule describe the Linux app on main.

Verified

test_detect_changed_paths.py passes with the new suite cases; actionlint and shellcheck clean; agent-docs and the docs check clean. This PR's own run exercises the new jobs.