feat: auto-update check via built-in scheduler task

[bug-ops]

Summary

Add automatic periodic update checking as a built-in scheduler task. Zeph queries GitHub Releases API for the latest version and notifies the user when a newer version is available.

Motivation

Users running long-lived Zeph instances (daemon mode, TUI) have no way to know a new release is available without manually checking. An opt-out periodic check solves this with minimal overhead.

Design

New TaskKind variant

Add TaskKind::UpdateCheck to zeph-scheduler alongside existing MemoryCleanup, SkillRefresh, HealthCheck.

Config

New section in config.toml under [agent] or as a dedicated table:

[agent]
auto_update_check = true  # default: true, set false to disable

[scheduler.tasks.update_check]
enabled = true
cron = "0 0 */6 * * *"  # every 6 hours, configurable

Environment override: ZEPH_AUTO_UPDATE_CHECK=false.

Implementation

1. Version check logic (in zeph-core):

   • GET https://api.github.com/repos/bug-ops/zeph/releases/latest via reqwest
   • Parse tag_name field, strip v prefix, compare with env!("CARGO_PKG_VERSION") using semver crate
   • Return Option<NewRelease { current, latest, url }> if newer version found

2. UpdateCheckHandler implementing TaskHandler trait:

   • Calls version check logic
   • On new version: logs info! message + sends notification to active channel (if available)
   • On same/older version or network error: logs debug!, does not notify user
   • Respects rate limiting (GitHub API: 60 req/h unauthenticated)

3. Registration in bootstrap:

   • When scheduler feature is enabled, register UpdateCheckHandler as a built-in task
   • Skip registration if auto_update_check = false

4. Notification format (channel message):

   New Zeph release available: v0.12.0 (current: v0.11.0)
   https://github.com/bug-ops/zeph/releases/tag/v0.12.0
   

What this does NOT do

• Does not download or install updates (user responsibility)
• Does not require GitHub auth token (uses public releases API)
• No menu integration yet (deferred to Epic: TUI Command Palette for agent management #572)

Dependencies

• semver crate (workspace dependency, add if not present)
• Existing: reqwest, serde, tracing
• Feature: scheduler (already optional)

Acceptance Criteria

• TaskKind::UpdateCheck variant added
• UpdateCheckHandler implements TaskHandler
• Config field auto_update_check (default true) + env override
• Scheduler task registered on bootstrap when enabled
• Notification sent to active channel on new release
• Graceful handling: network errors logged at debug, no user-facing noise
• Unit tests for version comparison logic
• --init wizard updated with auto-update option