docs: consolidate the four AI feedback validation docs; three describe pre-#3480 behaviour #3485

Problem

The AI feedback validation system is documented four times (about 1,900 lines), and three of the copies describe behaviour that changed in #3480 (PR for #3480):

• docs/WORKFLOW_AI_FEEDBACK_VALIDATION.md (updated in feat(ci): activate root workflows/ YAML as organisation reusable workflows #3480; the canonical reference)
• docs/AI_FEEDBACK_AUTOMATION_SETUP.md (603 lines): says the check always fails PRs, uses a pull_request_target trigger and checks: write, and points at root workflows/ai-feedback-validation.yml
• docs/AI_FEEDBACK_SYSTEM_SUMMARY.md (507 lines): describes the old 190-line two-job workflow
• docs/ai-feedback-response-tracking.md (433 lines): user guide; links ./.github/workflows/... relative to docs/, which resolves to a path that does not exist

Change

• Keep one reference (WORKFLOW_AI_FEEDBACK_VALIDATION.md) and one author guide (ai-feedback-response-tracking.md); fold anything unique from the other two into them and delete the rest.
• Describe the reusable workflow: workflow_call, the enforce input (warn-only by default), permissions, and the API read of FEEDBACK_RESPONSE.md (no PR checkout).
• Fix relative links.

Acceptance

• No doc describes the check as always failing, checks: write, or the root workflows/ path.
• Every link resolves.

Relates to #3480