[hyper-pm] GHA-PR-LLM-AC-006: Update one bot comment per PR instead of spamming

[nicnocquee]

Summary

On repeated pull_request events (e.g. synchronize), update the existing bot comment (hidden marker or API lookup) instead of creating a new top-level thread each time, so PRs stay readable.

Scope

• In scope: Find-or-create pattern for the review comment; update body for new head SHA; document any exception (e.g. GitHub API limitations).
• Out of scope: Resolving threaded review comments vs issue comments — pick one comment surface and document it.

Acceptance criteria

• On a second synchronize event for the same PR, the PR shows at most one visible top-level comment from this bot for this feature (per PRD REQ-007), or documented exception with maintainer rationale.
• Comment content reflects the current head SHA (idempotency per PRD §5).
• Automated test or integration script demonstrates find-and-update behavior where feasible; otherwise documented manual verification steps.

Dependencies

• Depends on: GHA-PR-LLM-AC-005
• Blocks: none

Notes

Coordinate with GHA-PR-LLM-AC-002 for guidance comments when no issue is linked — decide whether those also use the same single-comment mechanism.

PRD: https://github.com/hyperjumptech/mediapulse/blob/prds/pr-github-llm-ticket-acceptance.prd.md

{
  "hyper_pm_id": "01KP1FZ5ZYNJ9VT7PS4D83ADGX",
  "type": "ticket",
  "parent_ids": {
    "epic": "01KP1JW0X7SZ4SYS4N68M8ZDWA",
    "story": "01KP1JW25FFFPZAVZHQV4RZYNJ"
  },
  "depends_on": [
    "01KP1FZ5ZZVQ9RWMKHDYRESDME"
  ]
}