chore: add linters for plugins

[araujof]

Description

Adds lint and static analysis workflow for the plugins directory to maintain code quality and catch issues early in CI.

Changes

• New GitHub Actions Workflow: .github/workflows/lint-plugins.yml
  • Runs on push to main and PRs targeting main
  • Uses matrix strategy for parallel execution with fail-fast disabled (visibility into all failures)
• Linters & Tools Included:
  • Syntax & Format: yamllint
  • Python Code Quality: flake8, ruff, unimport, vulture
  • Standards: pylint (errors-only), interrogate (100% docstring coverage requirement)
  • Complexity Analysis: radon (cyclomatic complexity min C, maintainability index min B)
• Architecture (symmetrical to other linter workflows):
  • Each linter runs in isolated job for clear failure attribution
  • Installs project in editable mode with dev dependencies
  • Mirrors local CLI commands (no make wrapper)
  • Fast-failure isolation ensures one linter failure doesn't hide others

Impact

Establishes automated code quality gates for the plugin system, ensuring consistent standards and preventing technical debt in the plugins directory.