docs: sync contributing and add vision

[onutc]

TL;DR

"I want to add a VISION.md as well to acpx repo. The main vision for ACPX is that it's supposed to be a lightweight CLI for making any agent talk to another agent through the ACP protocol."

Summary

• Sync the fork with upstream main and copy the current OpenClaw CONTRIBUTING.md into this repo.
• Add a new root VISION.md for acpx focused on interoperability, a small core, robust configuration, and backend reuse.
• Keep a note in CONTRIBUTING.md that it was copied from OpenClaw.

Review focus

• Confirm the new vision captures the intended product boundaries for acpx.
• Confirm copying the upstream contribution guide remains the right maintenance approach.

Test plan

• Verified the fork is synced with upstream main.
• Verified the updated CONTRIBUTING.md content locally.
• Reviewed the new VISION.md locally.
• No code/tests run; docs-only change.