feat(services): cron-based scheduling with natural language conversion

[Mathews-Tom]

Summary

Replace the rigid interval_days scheduling with cron expression support and a natural language schedule converter, inspired by SivaRamSV/paaw's JobCreator NL-to-cron pattern. Users can now configure loop schedules as cron expressions ("0 9 * * 1-5") or natural language ("every monday at 9am", "weekdays", "every 3 days"), with automatic fallback to the existing interval_days for backward compatibility.

Schedule Resolution Chain

schedule field (string)
  ├─ Non-empty + valid cron syntax → use directly
  ├─ Non-empty + NL pattern match  → convert via nl_to_cron()
  ├─ Non-empty + unrecognized      → log warning, fall back ↓
  └─ Empty string                  → interval_days_to_cron(interval_days)

Supported Natural Language Patterns

Input	Cron Output
"daily" / "every day"	0 0 * * *
"weekly" / "every week"	0 0 * * 0
"monthly"	0 0 1 * *
"every monday" / "every fri"	0 0 * * 1 / 0 0 * * 5
"weekdays"	0 0 * * 1-5
"weekends"	0 0 * * 0,6
"every 3 days"	0 0 */3 * *
"every 4 hours"	0 */4 * * *
"daily at 9:30 AM"	30 9 * * *
"every monday at 3pm"	0 15 * * 1

Time extraction (at HH:MM AM/PM) combines with any day pattern.

Changes

New Files

• src/vaultmind/services/cron.py (172 lines) — CronSchedule frozen dataclass with is_overdue() and validate() methods. interval_days_to_cron() for integer-to-cron conversion. nl_to_cron() regex-based NL parser supporting 10+ patterns with time extraction. Pure pattern matching, no LLM cost
• tests/test_cron.py (205 lines) — 31 tests across 5 classes

Modified Files

• pyproject.toml — Added croniter>=2.0,<3 dependency
• uv.lock — Updated with croniter + pytz
• src/vaultmind/services/scheduler.py — Added cron_expr: str = "" field to ScheduledJob. is_overdue() delegates to CronSchedule.is_overdue() when cron_expr is set, falls back to timedelta comparison otherwise. Added resolve_cron_expr(schedule, interval_days) module-level function implementing the resolution chain
• src/vaultmind/config.py — Added insight_schedule, evolution_schedule, procedural_schedule fields to LoopsConfig (all default "" = derive from interval_days)
• config/default.toml — Expanded [loops] section with schedule fields alongside existing interval_days
• src/vaultmind/cli.py — Updated insight_loop, evolution_loop, and procedural_loop job creation to call resolve_cron_expr() and pass cron_expr to ScheduledJob

Backward Compatibility

• All existing interval_days configs continue to work — schedule="" triggers automatic conversion
• ScheduledJob interval field preserved; cron_expr is additive and takes priority only when non-empty
• Existing tests pass unchanged — ScheduledJob(name=..., interval=..., execute=...) still works
• ScheduledJob.legacy() factory unmodified

New Dependency

• croniter>=2.0,<3 — production-ready cron expression evaluator, widely used, lightweight (only pulls pytz)

Test plan

• 31 new tests in test_cron.py across 5 classes:
  • CronSchedule: validation, overdue checks with fixed/naive datetimes (5)
  • nl_to_cron: all NL patterns including time extraction, case insensitivity, unrecognized input (14)
  • interval_days_to_cron: day conversion, zero/negative edge cases (5)
  • resolve_cron_expr: explicit cron, NL conversion, empty/invalid fallback (4)
  • ScheduledJob cron integration: cron_expr priority, interval fallback, None last_run (3)
• All existing scheduler tests pass unchanged
• Full suite: 898/898 tests pass, 0 regressions
• ruff check — clean
• mypy --ignore-missing-imports — clean
• Manual: verify insight_schedule = "every monday at 9am" triggers correctly in bot
• Manual: confirm interval_days fallback produces identical behavior to pre-change