feat: add opt-in fleet health exit gate

[Tiyatrotist]

Adds an opt-in final gate to vitals_report. any_fail gates on the existing fleet failure count; regression gates only on postcheck hosts whose status changed from Pass at baseline to Fail. A configurable count threshold is supported, and the gate runs after rendering, archiving, and notifications so evidence is preserved before a non-zero exit.

Defaults keep current behavior unchanged. Variable reference, CI examples, and structural contract tests are included.

Closes #41.

Local pytest/Ansible execution was not run because the connected development machine is currently offline.

[coderabbitai]

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[pr-reviewer-bots]

Thanks for opening this PR. Assigned to @sameeralam3127. Labels added: documentation, size/medium. Checklist: 3/3 passed (see checks tab for details).

[sameeralam3127]

@Tiyatrotist: this PR and #102 both say Closes #41 and implement the same fleet health gate with different code (gate.yml and the tests differ). Please pick one and close the other, so review and CI effort go to a single branch.

Whichever you keep, this is what's blocking it right now:

#104: validate fails in tests/test_argument_specs.py

vitals_report: undocumented in argument_specs.yml:
  ['linux_vitals_fail_on_status', 'linux_vitals_fail_threshold_count']

• Add both variables to roles/vitals_report/meta/argument_specs.yml (type, default, choices for linux_vitals_fail_on_status, description). Every role default must be documented there.

#102: validate fails on ansible-lint

• Trailing whitespace at roles/vitals_report/tasks/main.yml:50
• CodeRabbit's finding on gate.yml:6: a non-numeric threshold such as "two" becomes 0 through the int filter instead of being rejected. Validate that it's a non-negative whole number before converting, and add a test for invalid values. The same applies to this PR's threshold handling.

In both cases the four required molecule (...) checks never ran, because validate has to pass first. Please run pre-commit run --all-files and pytest -q locally before pushing.