feat: add persistent startup command hook

[PabloZaiden]

Summary

• add --startup-command <command> for up and rebuild
• persist the command in workspace state so later external invocations and devbox arise reuse it
• execute the command with devcontainer exec after container setup, independently of bundled SSH
• add --no-startup-command to clear the persisted hook
• document the lifecycle and raw Docker restart behavior

This is intended for container-local services such as a Clanky worker. The command should be idempotent and daemonize long-running processes; raw docker restart is intentionally not covered, matching the existing Devbox SSH runner lifecycle.

Validation

• bun run typecheck
• bun run test
• bun run build

[Copilot]

🟡 Changes recommended

An invalid persisted startupCommand currently causes state loading to fail in a way that can prevent users from recovering/clearing the setting via the CLI.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

Adds a new, persisted “post-start” hook that runs a user-specified shell command inside the devcontainer after successful devbox up / devbox rebuild, with CLI flags to set or clear it and documentation/tests to cover the lifecycle.

Changes:

• Add --startup-command <command> / --no-startup-command to up and rebuild, persisting the value in workspace state.
• Execute the persisted command via devcontainer exec ... sh -lc <command> after container setup completes.
• Extend tests and README to document and validate persistence and execution behavior.

File summaries

File	Description
tests/runtime.test.ts	Adds unit coverage for startup-command script normalization/rejection.
tests/examples.test.ts	Adds an end-to-end simulated-host test ensuring the command runs and persists across up/rebuild, and can be cleared.
tests/core.test.ts	Adds CLI arg parsing and state-loading coverage for startup command options and persistence.
src/runtime.ts	Introduces runStartupCommand and buildStartupCommandScript and runs it via devcontainer exec sh -lc.
src/core.ts	Extends args/state schema with startupCommand and parsing helpers, including persisted-state validation.
src/cli.ts	Wires startup command selection/clearing into the up/rebuild lifecycle and persists it to state.
README.md	Documents the new flags, lifecycle behavior, and state file field.

Review details

• Files reviewed: 7/7 changed files
• Comments generated: 1
• Review effort level: Lite

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