pi: alarm extension, timed notes the agent sets for itself

An `alarm` tool sets, lists and cancels alarms: after a delay, at a
wall-clock time, or repeating (at most 12 fires, the last within 24 hours).
A fired alarm is a pi.sendMessage custom message, customType "alarm",
rendered as set by the agent rather than typed by the user, and wrapped in
fixed text the agent cannot drop: it is the agent's own earlier note, not
the user's, and grants no permission -- outward actions still need the
user's approval. /alarms lists and cancels.

An alarm due while pi is busy waits for the task to finish, held here
rather than in pi's queue, which drops a custom message on Esc. A run that
ended stopped -- Esc, an error, or the abort before /compact, /new, /resume
and /fork -- holds due alarms until a later run completes. Timers die with
the session; the tool is withdrawn outside the TUI.

Reviewed by Fable and GLM 5.3 Flash. Fixed from the review: unbounded
every_minutes overflowed setTimeout into 1 ms fires; a due alarm ran right
after Esc, or unseen on a session being replaced; stale repeat fires piled
up; the session_compact delivery never ran; impossible dates and times
skipped by a clock change were accepted; the note could forge the
wrapper's delimiters.

Declines dot request 20261021-220352-work-laptop-c46033 (sz-agent-alarm, a
skill typing into the agent's tmux pane) in favour of this: a custom message
is recorded and shown as the agent's, needs no screen-scraping, and loads
only in pi.

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