fix: provide executorContextConstraints to plan-slice template (#677)

The plan-slice.md template declares {{executorContextConstraints}} but
buildPlanSlicePrompt() never passed this variable, causing loadPrompt()
to throw: 'template declares {{executorContextConstraints}} but no value
was provided.'

Add formatExecutorConstraints() that uses the budget engine
(computeBudgets + resolveExecutorContextWindow) to generate the
executor context constraints block with task count ranges and inline
context budgets based on the configured executor model's context window.

Pass the formatted string to loadPrompt() as executorContextConstraints.

Closes #677

src/resources/extensions/gsd/auto-prompts.ts

@@ -15,11 +15,41 @@ import {
   relMilestoneFile, relSliceFile, relSlicePath, relMilestonePath,
   resolveGsdRootFile, relGsdRootFile,
 } from "./paths.js";
-import { resolveSkillDiscoveryMode, resolveInlineLevel } from "./preferences.js";
+import { resolveSkillDiscoveryMode, resolveInlineLevel, loadEffectiveGSDPreferences } from "./preferences.js";
 import type { GSDState, InlineLevel } from "./types.js";
 import type { GSDPreferences } from "./preferences.js";
 import { join } from "node:path";
 import { existsSync } from "node:fs";
+import { computeBudgets, resolveExecutorContextWindow } from "./context-budget.js";
+
+// ─── Executor Constraints ─────────────────────────────────────────────────────
+
+/**
+ * Format executor context constraints for injection into the plan-slice prompt.
+ * Uses the budget engine to compute task count ranges and inline context budgets
+ * based on the configured executor model's context window.
+ */
+function formatExecutorConstraints(): string {
+  let windowTokens: number;
+  try {
+    const prefs = loadEffectiveGSDPreferences();
+    windowTokens = resolveExecutorContextWindow(undefined, prefs?.preferences);
+  } catch {
+    windowTokens = 200_000; // safe default
+  }
+  const budgets = computeBudgets(windowTokens);
+  const { min, max } = budgets.taskCountRange;
+  const execWindowK = Math.round(windowTokens / 1000);
+  const perTaskBudgetK = Math.round(budgets.inlineContextBudgetChars / 1000);
+  return [
+    `## Executor Context Constraints`,
+    ``,
+    `The agent that executes each task has a **${execWindowK}K token** context window.`,
+    `- Recommended task count for this slice: **${min}–${max} tasks**`,
+    `- Each task gets ~${perTaskBudgetK}K chars of inline context (plans, code, decisions)`,
+    `- Keep individual tasks completable within a single context window — if a task needs more context than fits, split it`,
+  ].join("\n");
+}
 
 // ─── Inline Helpers ───────────────────────────────────────────────────────
 
@@ -603,6 +633,9 @@ export async function buildPlanSlicePrompt(
 
   const inlinedContext = `## Inlined Context (preloaded — do not re-read these files)\n\n${inlined.join("\n\n---\n\n")}`;
 
+  // Build executor context constraints from the budget engine
+  const executorContextConstraints = formatExecutorConstraints();
+
   const outputRelPath = relSliceFile(base, mid, sid, "PLAN");
   return loadPrompt("plan-slice", {
     workingDirectory: base,
@@ -613,6 +646,7 @@ export async function buildPlanSlicePrompt(
     outputPath: join(base, outputRelPath),
     inlinedContext,
     dependencySummaries: depContent,
+    executorContextConstraints,
   });
 }