Update agent guidelines

[Pasarus]

Description

This pull request updates agents.md to provide a comprehensive guide and set of standards for AI agents working on the plotting-service repository.

Summary of Guidelines in agents.md:

• Project Overview & Structure: Documentation of core technologies (FastAPI, h5grove, Pillow) and the repository's package layout.
• Development Workflow:
  • Environment Setup: Standardized setup using Conda (recommended) or Pip/Venv.
  • Execution: Commands for running the API (uvicorn) and tests (pytest).
  • Conventions: Branch naming rules (snake_case, no prefixes) and issue creation using the organization's template.
  • Pull Request Process: Requirements for local validation (pytest, ruff, mypy), PR title formatting (normal grammar), and description structure (linking issues).
  • Dependabot Workflow: A 6-step process for listing, reviewing, validating (with specific local/remote checks), and merging dependency updates, including handling branch protection.
• Agent Guidelines:
  • Path Handling: Standards for pathlib.Path and environment variables (CEPH_DIR, IMAT_DIR).
  • Authentication: Guidance on JWT and API key middleware.
  • Quality & Testing: Requirements for test coverage, using existing test structures, and adherence to Ruff formatting.

These updates consolidate established practices and recent workflow improvements into a single reference for agents.