fix(markdown-renderer): collapse 3+ consecutive newlines in slice plan

Task descriptions in slice plans sometimes contained double-blanks
(model emits multi-paragraph content with its own paragraph padding,
which survives normalizeMarkdownBlockSpacing's heading-only padding
logic). The double blanks tripped MD012/no-multiple-blanks in
pre-execution checks and blocked the autonomous loop at the
execute-task phase.

Live observation today: SF iter2 completed research-slice and
plan-slice for M006/S01 cleanly, then pre-execution checks failed on
the generated S01-PLAN.md with two MD012 violations at lines 99-100
and 126-127 (both inside task description paragraphs). SF paused
"Autonomous mode paused (Escape)" awaiting user — autonomous loop
stalled.

auto_fix_check_failures: true in prefs should have handled this but
doesn't run for files under .sf/milestones/ (separate bug worth
filing). Fix at source: collapse runs of 3+ newlines to 2 in the
final rendered slice plan. Surgical, no semantic change, defensive
against future model-quirks too.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/resources/extensions/sf/markdown-renderer.js

@@ -587,7 +587,14 @@ function renderSlicePlanMarkdown(slice, tasks, gates = []) {
 		}
 	}
 	lines.push("");
-	return `${lines.join("\n").trimEnd()}\n`;
+	// Collapse 3+ consecutive newlines down to 2. Task descriptions sometimes
+	// contain double blanks (model-emitted multi-paragraph content + paragraph
+	// padding) that survive normalizeMarkdownBlockSpacing's heading-only logic.
+	// Without this, the rendered S0?-PLAN.md trips MD012/no-multiple-blanks in
+	// pre-execution checks and blocks the autonomous loop at execute-task.
+	// auto_fix_check_failures was supposed to handle this but doesn't run for
+	// .sf/milestones/ files — quicker to emit clean markdown at the source.
+	return `${lines.join("\n").trimEnd()}\n`.replace(/\n{3,}/g, "\n\n");
 }
 export async function renderPlanFromDb(basePath, milestoneId, sliceId) {
 	const slice = getSlice(milestoneId, sliceId);