Make --auto-close today a deadline, and name the real error

[monperrus]

Fixes #83.

What was wrong

shell help reference lists today among the accepted --auto-close dates, but a bare day keyword resolved to midnight. Midnight today has passed whenever the command runs, so --auto-close today was rejected at every hour of every day — there was no time at which the documented form could work.

The rejection also misreported itself. The unquoted form (--auto-close <value> <command>) tries each prefix of the following arguments and, when no prefix parsed, discarded the parse error and called the value invalid. So --auto-close 2020-01-01 complained about grammar for a date it had read perfectly well, while the quoted --auto-close=2020-01-01 already said the deadline must be in the future.

What changed

• Bare today now means the end of today, 23:59:59 local. The last second rather than the last minute, so the form still works while 23:59 is on the clock — otherwise the fix would leave a minute a day where it failed again.
• Bare tomorrow is unchanged. The start of that day is still ahead, so it was already a deadline; touching it would change behaviour people already rely on.
• normalizeAutoCloseArguments keeps the first token's own error and returns it when no prefix parses, so the unquoted and quoted forms now agree on why a value was refused.
• shell help reference states what each bare keyword means instead of leaving it to be inferred.

User-visible behaviour

$ shell --auto-close today sleep 600
  Closes     2026-09-11T23:59:59+02:00, or when the task exits

$ shell --auto-close 2020-01-01 sleep 600
shell: auto-close deadline must be in the future

Before, the first printed shell: invalid auto-close value "today" and exited 2, and the second reported the same grammar error.

Security implications

None. --auto-close can only bring a session's deadline earlier than the task's own exit, and this change adds one accepted spelling of a deadline that is at most 24 hours out. It cannot extend a session, and it does not touch the relay, the session protocol, or the encryption path.

Regression tests cover the bare keywords, the unquoted form, that today stays a future deadline at every hour including 23:59:30, and that a past date reports a past deadline. go test -race ./..., go vet ./... and a gofmt check all pass.

🤖 Generated with Claude Code

[TeoSlayer]

Useful feature, will amend docs to include it. Good on my end.

[TeoSlayer]

Integrated into v0.12.1 via #118 after resolving the changelog conflict against current main. The auto-close behavior and regression tests are included. Thank you.