Support multiple environment managers in AGENTS.md activation instructions

[Copilot]

The environment activation instructions in AGENTS.md were hardcoded to conda activate CausalPy, but developers use different tools (micromamba, pixi, etc.).

Changes

• Environment activation: Provide commands for conda, micromamba, and pixi instead of assuming conda
• Tool detection: Add command to detect available environment managers: which conda || which micromamba || which pixi
• Documentation reference: Link to environment.yml for consistency

Example

- **Activation required**: Always activate the `CausalPy` environment before executing any Python commands, tests, or scripts
  - Try one of the following commands based on your environment manager:
    - `conda activate CausalPy` (for conda)
    - `micromamba activate CausalPy` (for micromamba)
    - `pixi shell` (for pixi)
  - If unsure which tool you're using, try running `which conda || which micromamba || which pixi` to detect available tools

This approach avoids requiring local configuration files while supporting different developer workflows.

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✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.

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📚 Documentation preview 📚: https://causalpy--583.org.readthedocs.build/en/583/