[Documentation] CI host maintenance docs still say weekly after timer changed to daily

[Osraka]

Problem

The current scripts/ci_host_maintenance.sh installs a daily systemd timer:

Description=Daily Verity CI host maintenance
OnCalendar=*-*-* 04:30:00

However, the operator-facing text still describes the timer as weekly:

• scripts/ci_host_maintenance.sh help: Install and enable a weekly systemd timer
• scripts/README.md: Install weekly cache, journald, and Docker image cleanup.
• docs/CI_RUNNERS.md: Each CI host should have the weekly Verity maintenance timer installed.

Reproduction

1. Run scripts/ci_host_maintenance.sh install-systemd.
2. Inspect the generated unit with systemctl cat verity-ci-host-maintenance.timer.
3. It schedules at *-*-* 04:30:00 (daily), while the help and operator docs say weekly.

Why this matters

This is an operationally relevant documentation regression introduced when the maintenance cadence changed after the CI disk-exhaustion incident. An operator following the docs can form the wrong expectation about cache cleanup cadence and disk growth.

Proposed narrow fix

• Change the three stale descriptions to daily.
• Extend scripts/test_ci_infra_maintenance.py to assert the help/docs wording remains aligned with the generated timer.
• Do not change the timer or any cleanup behavior.

I'd be happy to prepare this as a small documentation/test PR if this scope matches the project's preference.