add scheduled tasks as CronJobs

[atkrad]

Summary

• Add on: schedule so a task renders as a Kubernetes CronJob in the release.
• Validate cron at spec load. Default Forbid concurrency and Etc/UTC. When timeout is omitted, the CronJob gets a 1h cluster deadline; deployah run does not.
• Cap task names at 30 characters so CronJob names stay under the 52-character Kubernetes limit.

Related

Fixes #35

Test plan

• Unit tests added/updated
• Scenario under scenarios/ (if behavior changes)
• nix run .#lint / pre-commit clean
• Manual smoke (command + expected result), if user-facing

Labels

• One of: kind/feature, kind/bug, kind/docs, kind/chore
• Add breaking-change if this breaks existing CLI or config behavior
• Add skip-changelog for internal-only PRs that should not appear in notes

Checklist

• Title is short and imperative (matches commit style)
• Docs / CLI help updated when user-facing (README.md, docs/cli/)
• No secrets or local-only paths in the diff

[codecov]

Codecov Report

❌ Patch coverage is 90.17341% with 17 lines in your changes missing coverage. Please review.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
internal/helm/task.go	86.36%	15 Missing ⚠️
internal/helm/generate.go	83.33%	1 Missing ⚠️
internal/spec/task_validate.go	97.82%	1 Missing ⚠️

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