ci(linux): add the Linux workflow and weekly dispatcher to main

[datlechin]

Why

GitHub runs a schedule only from the default branch, and main has no Linux workflow. So the Linux app's Monday run never fires: the full Docker matrix, the TLS suites and the fork drift checks.

Change

• build-linux.yml comes to main unchanged from linux. On main its detect job finds no Linux tree and every build job skips. On the schedule, weekly-dispatch starts the workflow on the branch the LINUX_BRANCH variable names, linux when unset.
• .claude/rules/ci.md says why the file is on main.
• When linux/ merges into main, the file is already identical, so that merge brings no conflict.

Verified

actionlint and zizmor are clean, and every build job is gated on detect. agent-docs passes.