feat(cron): usercron hot-reload via external cronjob.toml

[chaodu-agent]

Summary

Add usercron support and consolidate all cron config under [cron].

BREAKING CHANGE: [[cronjobs]] → [[cron.jobs]]

Closes #628

Flow

                         config.toml                        $HOME/cronjob.toml
                    ┌──────────────────┐                 ┌──────────────────────┐
                    │ [cron]           │                 │ [[jobs]]             │
                    │ usercron_enabled │                 │ schedule = "* * * *" │
                    │   = true         │                 │ channel  = "123..."  │
                    │ usercron_path    │                 │ message  = "ping"    │
                    │   = "cronjob.toml│"                └──────────┬───────────┘
                    │                  │                            │
                    │ [[cron.jobs]]    │                   Agent writes here
                    │ (baseline jobs)  │                   anytime (mobile/CLI)
                    └────────┬─────────┘                           │
                             │                                     │
                    ┌────────▼─────────┐                           │
                    │  OAB Scheduler   │◄──────────────────────────┘
                    │  (ticks every    │   check mtime every tick
                    │   1 minute)      │   reload if changed
                    └────────┬─────────┘
                             │
              ┌──────────────┼──────────────┐
              │              │              │
     baseline jobs    usercron jobs    should_fire()?
     (immutable)      (hot-reload)         │
              │              │         ┌────▼────┐
              └──────────────┘    no── │ match?  │ ──yes──► fire_cronjob()
                                      └─────────┘          → send message
                                                            → create thread
                                                            → agent processes

Config

[cron]
usercron_enabled = true
usercron_path = "cronjob.toml"   # resolves to $HOME/cronjob.toml

[[cron.jobs]]
schedule = "0 9 * * 1-5"
channel = "123456789"
message = "daily summary"

External cronjob.toml uses [[jobs]] (same fields).

Changes

• config.rs: New CronConfig struct with usercron_enabled, usercron_path, jobs; [[cronjobs]] → [[cron.jobs]]
• cron.rs: load_usercron_file(), parse_job_list(), mtime-based hot-reload; external file uses [[jobs]]
• main.rs: Gate usercron on [cron].usercron_enabled, resolve relative paths from $HOME
• config.toml.example: All cron config under [cron]
• docs/cronjob.md: Full rewrite with ASCII flow diagram
• docs/config-reference.md: Consolidated [cron] section

Testing

• 5 new unit tests for usercron file loading/validation
• All 166 existing tests pass

Discord Discussion: https://discord.com/channels/1491295327620169908/1491365150664560881/1498838230018298026

[chaodu-agent]

Discord Discussion: https://discord.com/channels/1490282656913559673/1498821935059042384