Add on-call rotation feature

[ewels]

Summary

• Adds a complete on-call rotation system managed via /nf-core on-call slash commands, backed by the existing DynamoDB table
• All on-call commands are restricted to @core-team members (permission checked centrally in the router)
• Includes a background async scheduler that auto-extends the 8-week rolling roster, sends reminders (assignment, 1-week-before, daily at 8am user timezone), and posts Monday announcements to #core
• Round-robin assignment uses @core-team usergroup membership — no separate member list to maintain
• Fixes pre-existing mypy error in admin.py:669 that was failing CI on main

Commands

All commands require @core-team membership.

Command	Description
on-call list	Show the upcoming ~8 weeks of assignments
on-call me	Show the caller's upcoming on-call dates
on-call switch [YYYY-MM-DD]	Swap on-call weeks with another person, DMs both parties
on-call skip	Skip next on-call week, assigns a replacement via round-robin
on-call unavailable <start> <end>	Mark unavailable for a date range, auto-skips overlapping weeks
on-call reboot	Wipe and rebuild the entire schedule from scratch

New files

• src/nf_core_bot/db/oncall.py — DynamoDB CRUD (roster, round-robin state, unavailability, reminder tracking)
• src/nf_core_bot/commands/oncall/ — 6 command handlers + shared helpers
• src/nf_core_bot/scheduler/oncall_jobs.py — Background scheduler (roster extension, reminders, cleanup)
• tests/test_oncall_db.py, tests/test_oncall_commands.py, tests/test_oncall_scheduler.py — 84 new tests

Modified files

File	Change
app.py	Launches scheduler as background task (with strong reference to prevent GC)
router.py	Routes on-call subcommands; ack() + is_core_team() checked centrally before dispatch
help.py	On-call help text, all commands marked admin-only
admin.py	Fix pre-existing mypy error on line 669 (typed org_lists slice)
CLAUDE.md	Updated with on-call architecture, DynamoDB key patterns, and command docs

DynamoDB key patterns

PK	SK	Purpose
ONCALL#<YYYY-MM-DD>	ROSTER	Weekly roster entry
ONCALL_META	ROUND_ROBIN	Round-robin state
ONCALL_META	REMINDERS#<YYYY-MM-DD>	Reminder tracking per week
ONCALL_UNAVAIL#<user_id>	<start>#<end>	Unavailability range

Architecture notes

• Permission model: All on-call commands gated by @core-team membership check in _route_oncall(). ack() called once centrally; handlers do not receive ack.
• Scheduler: Background asyncio task (60s loop). Monday jobs (roster extension, cleanup) fire once per week via _last_weekly_run guard. Daily reminders respect user Slack timezone.
• Round-robin: Picks person with oldest last_assigned date. queue_front list gives priority to people who previously skipped. When all members exhausted, cycles through again.
• Reboot: Deletes all roster entries + resets round-robin state, then rebuilds. Useful for initial deployment or when core team membership changes.

Quality

• 356/356 tests pass (84 new + 272 existing)
• Ruff: clean
• Mypy: 0 errors (including fix for pre-existing admin.py issue)