docs: add 'good first issue' contributor path to CONTRIBUTING

[alexar76]

Why

Newcomers land on a large monorepo and bounce. A short contributor path lowers that barrier and feeds goodfirstissue.dev.

Task

1. Ensure CONTRIBUTING.md (or create it) has a Good first issues section linking to:
   https://github.com/alexar76/aicom/labels/good%20first%20issue
2. Add 5–8 lines: how to clone, which packages need pip install -e, how to run a single test file.
3. Link the three flagship satellites: metis, aimarket-mcp, and profile try-it block.

Acceptance

• CONTRIBUTING links the label filter
• One copy-paste command to run a smoke test
• No ecosystem dump — keep under ~40 lines for this section