chore: Add copilot instructions and skills for building and testing plugins

[mofojed]

Tested by running the following prompts:

1. "Run tests for deephaven.ui"

• Without the skills, keeps trying to run using pytest, with the wrong .venv, or in the wrong directory
• With the skills, it runs the tests using tox correctly the first time

2. "Build deephaven.ui and start a server"

• Sometimes it would start up fine, other times it would end up running npm run docker instead
• With the skill, it sources the .venv and runs plugin_builder

3. "Run the JS tests for plotly-express"

• It took a while, but did eventually run them correctly without the skill
• With the skills it ran them faster

[dsmmcken]

1. What testing did you do? I assume you tried asking for various things and the skills helped?

2. I think copilot-instructions.md should be an AGENTS.md, that seems to be the preferred approach now no?