[FEATURE] Coverage Phase 1: Wire observability into CI

[sergio-sisternes-epam]

Is your feature request related to a problem? Please describe.
CI has no coverage measurement. Contributors cannot see coverage impact
of their changes, and regressions land silently.

Describe the solution you'd like
Wire pytest-cov into both CI pipelines (unit and integration) with
report-only output to $GITHUB_STEP_SUMMARY. No fail_under gate --
this phase establishes observability only.

Changes

1. Add pytest-cov to dev dependencies
2. Add [tool.coverage.run] and [tool.coverage.report] to pyproject.toml
   • source = ["src/apm_cli"], branch = true, parallel = true
   • Exclude patterns: pragma: no cover, TYPE_CHECKING, NotImplementedError
3. Update ci.yml unit test step with --cov flags
4. Add coverage summary step writing markdown table to $GITHUB_STEP_SUMMARY
5. Update ci-integration.yml sharded test step with --cov flags
6. Add per-shard coverage artifact upload
7. Add fan-in job to download, merge, and render integration coverage summary
8. No fail_under gate -- observability only

Describe alternatives you've considered

• Using Codecov or Coveralls -- rejected to avoid third-party dependency;
  GitHub-native $GITHUB_STEP_SUMMARY keeps everything in-repo.

Additional context
Parent: #1398 -- Phase 1 of the progressive coverage strangler-fig ratchet.

Acceptance criteria

• Unit coverage summary appears in CI job summary (markdown table)
• Integration coverage summary appears in fan-in job summary
• Both summaries include overall % and collapsible lowest-coverage files
• No PR is blocked by coverage -- report only