ci(linux): run Flatpak, MSRV and packages nightly, cap docker tests

[datlechin]

Why

A Linux PR or main push started 25 Build Linux jobs, plus 6 Linux packages jobs on a main push. GitHub Free runs 20 jobs at once across the account, so Linux runs held back macOS and iOS ones.

Change

• Flatpak Devel (both arches) and Minimum Rust version run only on schedule and dispatch.
• Build Linux's cron goes from weekly to nightly; a scheduled run still takes every job.
• Docker tests matrix runs 3 entries at a time (max-parallel: 3).
• Documentation links (lychee) moves into Fast checks, on the bare checkout; the docs job is gone.
• linux-packages.yml drops its push trigger and runs nightly instead; its PR trigger, dispatch and workflow_call stay.
• Docs that stated these triggers now point to each workflow's on:.

Verified

actionlint, zizmor 1.30.1 offline, lychee 0.24.2 over linux/**/*.md, check-doc-symbols.sh and verify.sh agent-docs pass locally.