brigade: next prints the steps one wake owes and whether to reply

[creedants]

Adds brigade.py next, a read-only command that prints what one wake of the coordinator owes.

Given the wake (a liveness check, a worker's report, a review verdict, a merge, a morning or evening report, and so on) it prints the steps as run <command>, see <words>, and if <condition>: <action> lines, then one reply: yes|no (reason) line that follows the skill's reply rules for the store's reporting level.

• It writes nothing: no table, log row, lease, or file, on any path.
• Every run line is a command the tool accepts as printed on the store as it stands.
• What only the coordinator can see is printed as a condition, not a fact.
• A cell that should hold a number and does not gives a warning and the sheet still prints.

One change to existing behavior. A log timestamp with no timezone is now read as UTC, so watch no longer fails on a hand-edited stamp of that kind. Stores whose timestamps carry a timezone print the same as before.

Known limits.

• A malformed row in the items, goals, tickets, log, or decisions table makes next refuse with the same one-line message the other commands print.
• A store with a bad restaurant.json still fails the way status does.
• No fire line per ticket; the sheet points at startable.
• It reads no thread, verdict file, or pull request. A failed send is seen only when --unsent is passed.
• watch still fails on a timebox that is not a number.
• The usage line shows the wake as required, though a bare next is accepted.
• The skill text does not use next yet.

Reviewed at 6a044640.