feat(cron): add manual exec command for existing jobs

[xukp20]

Summary

Add a manual cron exec entrypoint so an existing cron job can be triggered immediately by job ID.

This reuses the existing cron execution path instead of introducing a separate ad-hoc runner.

Fixes #263

What changed

• add CronScheduler.TriggerJob(id) for manual execution of existing jobs
• add /cron exec <id> in chat command handling
• add cron card Run Now action button
• add local API endpoint POST /cron/exec
• add CLI command cc-connect cron exec <id>
• update cron help text and i18n strings
• add regression tests for scheduler, card action, text command, API, and CLI usage

Why

Today cron jobs can be created, listed, edited, enabled, disabled, muted, and deleted, but they cannot be run immediately.

This makes testing and one-off triggering awkward because users have to wait for the next scheduled time.

cron exec solves that by letting an existing job run once on demand while preserving its configured project, session, prompt/exec mode, timeout, and
other execution settings.

Validation

• go test ./core -run 'Cron|ExecuteCronJob|HandleCronExec' -v
• go test ./cmd/cc-connect -run 'Cron' -v
• $(go env GOPATH)/bin/staticcheck ./core/... ./cmd/cc-connect/...
• go build ./...
• go test ./...

[chenhg5]

LGTM. Useful feature for cron job testing.

Review notes:

• ✅ CI passes
• Adds cron exec <id> command for manual trigger
• Includes CLI, API endpoint, card action
• Good test coverage
• Fixes issue #263

Approved for merge.