fix(orchestrator): deduplicate '## Plan' heading in PR body

[Maxtanh-Meta]

Problem

buildPRBody unconditionally prepends a ## Plan heading before inserting job.PlanText. However, the planning agent's output often already starts with its own ## Plan heading (since the prompt says "any structure you like"). This produces PR descriptions with a duplicate ## Plan heading — the first one containing all the plan content, and the second one empty right before ## Validation.

Example: https://github.com/Maxtanh-Meta/one-minute-games/pull/39

Fix

Check if PlanText already starts with ## Plan before adding one. If the agent included the heading, we skip ours; if not, we add it as before.

Test

Added TestBuildPRBodyNoDuplicatePlanHeading covering both cases (agent includes heading vs. doesn't).

[Maxtanh-Meta]

Code Review (via Codex)

Overall: Approve with minor suggestion

Main Concern

strings.HasPrefix(planText, "## Plan") is a loose prefix match — it also matches headings like ## Planning, ## Planet scale concerns, etc. Those would incorrectly skip adding the canonical ## Plan heading, producing a PR body missing its expected top-level section.

Suggested fix: tighten the check to an exact heading boundary:

if !strings.HasPrefix(planText, "## Plan\n") && planText != "## Plan" {
    b.WriteString("## Plan\n\n")
}

Additional Notes

• Whitespace-only PlanText: A value like " \n" still passes the != "" guard and would emit an empty ## Plan section. Since TrimSpace is already computed, consider skipping the section when trimmed text is empty.
• Test looseness: strings.Count(body, "## Plan") counts substrings inside ## Planning too. Prefer checking exact heading lines.
• Suggested test case: Add a case for "## Planning\n..." to verify the canonical heading is still added.

Verdict

The fix is correct for the common case and the tests cover the main regression well. Just needs the prefix match tightened. 👍