Add non-working mode and workday boundary rules to /timebox

[cristoforows]

What

Extends /timebox with a third day type and sharper working-day rules, per a grilled-out spec.

Non-working day mode

• New 🌴 Day off option alongside Office/WFH.
• Picking it asks for a start time before collecting tasks: 24-hour free text (900→09:00, 1330→13:30, 0930, colon form 9:00 also accepted; anything ambiguous/out-of-range re-asks), or a Start 09:00 default button.
• Non-working days get no commute, no work window, and no focus filling — just tasks around lunch/dinner from the chosen start.

Workday boundary rules (office/wfh)

New TIMEBOX_WORK_START / TIMEBOX_WORK_END (soft) / TIMEBOX_WORK_END_HARD bound a work window inside the day. The planner now:

• Classifies each task work vs non-work by inference.
• Keeps work tasks inside 09:00–17:00, spilling to 18:00 only when needed (avoiding 17–18), never past the hard end — overflow is dropped and reported.
• Caps non-work at ≤60 min total inside the work window; the rest goes to the evening.
• Fills empty gaps with placeholder focus slots: Work Focus (90m) + Break (15m) in 09:00–17:00, Personal Focus + Break in 17:00–23:30.

Other

• Day end extended to 23:59 so the evening is plannable.
• Dropped tasks now surface a clear "⚠️ N task(s) couldn't be scheduled (ran out of time)" notice in both the text-only and calendar-published replies.

Implementation notes

• One system prompt, _MODE_BLOCK fragment-switches the commute line and the work-window rules are injected only for working days — no prompt triplication.
• DayConfig gains the work window; _day_config(start_override=…) threads the non-working start time in without mutating the frozen config.
• Generated Focus/Break events carry the existing source=timebox tag, so redo's clear-then-write removes them cleanly.

Testing

• tests/test_scheduler.py updated + new cases: work-window/focus rules present for office/wfh, absent for nonworking; dropped-notice wording. 17 passed locally.