README: running an Architect run on a schedule #249

Parent

#237

What to build

A README section, under the architect documentation, on running an Architect run on a schedule. thirdshift has no scheduler of its own: the operating system's scheduler runs the ordinary command. The section gives a complete crontab entry to paste and edit:

PATH=/home/you/.local/bin:/home/you/.cargo/bin:/usr/local/bin:/usr/bin:/bin
0 2 * * * cd ~/repos/thirdshift && thirdshift architect base main >> ~/.thirdshift/logs/architect-cron.log 2>&1

and explains it in a few sentences, checked against how the commands from #246, #247 and #248 actually behave once they have landed.

The recipe can't be run in CI or by an agent. Write it for a Linux machine with cron, and say so; a person tries the line on a real machine when reviewing the Spec PR.

Acceptance criteria

• The README has the crontab entry, with placeholders for the user's own paths.
• It says why PATH is set: cron doesn't read the shell profile, and thirdshift, claude, gh, git and the build tools must be found.
• It says why the line uses base main: so the branch checked out in the clone doesn't matter.
• It says to use one line per repository, at different hours.
• It says what the log file is for: failures before the Run notification's checks, such as a broken User config or a missing Resend key, show up only there.
• It says claude and gh must already be logged in for the user the schedule runs as.
• It shows no-merge and --plan-only as the cautious variants, and says the command's flags and the User config decide what a pass does, as for a hand-typed run.
• It says when a scheduled pass is skipped (another Architect run still running, or an open Architect plan), that a skip exits 0, and that with Run notifications on, every night sends one email.
• One sentence says any scheduler that runs the command works, and that a systemd user timer needs lingering on to fire while the user is logged out.
• thirdshift help points to running it on a schedule in a line or two, without repeating the recipe.
• Nothing in the README still tells the user to avoid running two Architect runs at once by hand.

Blocked by

• thirdshift architect base <branch>: an Architect run names its Base branch #246
• One Architect run per repository at a time, with a skipped outcome #247
• The architect-plan label, and skipping an Architect run while an Architect plan is open #248