feat: add preflight hook to run commands before dip compose

[LeipeLeon]

Summary

• Adds a new top-level preflight: key to dip.yml — an array of shell commands run before any dip compose ... subcommand (including dip up). A non-zero exit aborts before Docker Compose runs.
• Mirrors the existing provision pattern: sequential exec_subprocess calls with panic: true, env interpolation, default [].
• Use cases: verifying credentials are present, required files exist, env vars are set, etc.

Example

preflight:
  - ./bin/check-credentials
  - test -f .env

Test plan

• New specs in spec/lib/dip/commands/preflight_spec.rb cover no-op, sequential execution, and env interpolation
• config_spec.rb asserts the preflight accessor exists and defaults sanely
• Schema validation: preflight is a typed array of strings in schema.json
• README updated with a usage example

🤖 Generated with Claude Code

[bibendi]

Thanks for the PR! The feature is well implemented — clean code, good tests, and the Provision-like pattern fits right in.

One architectural concern:

Preflight currently only runs in Commands::Compose#execute, which means it doesn't cover:

• dip ktl <cmd> — direct kubectl path
• dip run <cmd> with KubectlRunner or LocalRunner

DockerComposeRunner gets preflight transitively (it calls Compose#execute), but that's implicit and fragile.

I'd suggest moving preflight up to three entry points instead:

1. Remove the preflight call from Commands::Compose#execute
2. Add it to Dip::CLI#compose — covers dip compose, dip up, dip build, dip stop, dip down
3. Add it to Dip::CLI#ktl — covers dip ktl
4. Add it to Dip::Commands::Run#execute — covers all runners (DockerComposeRunner, KubectlRunner, LocalRunner)

This avoids double execution while covering every code path cleanly. Dip.env is available at all three points — config is already loaded by the time they run.

What do you think?

[LeipeLeon]

addressed your feedback, good call!

entry point for checking is a lot cleaner now.

btw: thnx for this great tool!

[bibendi]

Looks good to me! Thank you!
I'll merge it tomorrow