fix(work): emit completion signal to stop sandcastle agent loops

Sandcastle re-invokes agents up to maxIterations even when the work is
already done — the decomposer was looping 4x re-writing the same epic
on every dispatch. Two halves to the fix:

- Pass completionSignal: "<promise>COMPLETE</promise>" explicitly on
  all three run() calls (decompose, implementer, reviewer). Makes the
  contract visible alongside maxIterations instead of relying on
  sandcastle's default.
- Append a "Signal completion (required)" section to each prompt
  telling the agent to emit the literal marker as its final line when
  the work is genuinely done, plus a "do NOT emit if..." list to
  discourage premature signaling.

scripts/work/decompose.mjs

@@ -166,6 +166,11 @@ export async function executeDecompose(prdId, prdPath, prdText) {
       // context, write epic + stories, commit); 10 iterations is enough
       // room. Tune via env SANDCASTLE_DECOMPOSE_ITERATIONS.
       maxIterations: Number(process.env.SANDCASTLE_DECOMPOSE_ITERATIONS ?? 10),
+      // Stop iterating the moment the agent emits this marker. Without it,
+      // sandcastle re-invokes the model up to maxIterations even when the
+      // work is already done — the prompt instructs the agent to emit
+      // <promise>COMPLETE</promise> on its final line.
+      completionSignal: "<promise>COMPLETE</promise>",
     });
   } catch (e) {
     console.error("✗ Decomposer dispatch failed:", e.message);

scripts/work/dispatch.mjs

@@ -237,6 +237,11 @@ async function executeDispatch() {
       maxIterations: Number(
         process.env.SANDCASTLE_IMPLEMENTER_ITERATIONS ?? 30,
       ),
+      // Stop iterating the moment the agent emits this marker. Without it,
+      // sandcastle re-invokes the model up to maxIterations even when the
+      // work is already done — the prompt instructs the agent to emit
+      // <promise>COMPLETE</promise> on its final line.
+      completionSignal: "<promise>COMPLETE</promise>",
     });
   } catch (e) {
     console.error("✗ Implementer dispatch failed:", e.message);
@@ -290,6 +295,11 @@ async function executeDispatch() {
     // Smaller surface than the implementer; 10 iterations is plenty.
     // Tune via env SANDCASTLE_REVIEWER_ITERATIONS.
     maxIterations: Number(process.env.SANDCASTLE_REVIEWER_ITERATIONS ?? 10),
+    // Stop iterating the moment the agent emits this marker. Without it,
+    // sandcastle re-invokes the model up to maxIterations even when the
+    // decision has already been returned — the prompt instructs the agent
+    // to emit <promise>COMPLETE</promise> on its final line.
+    completionSignal: "<promise>COMPLETE</promise>",
   });
   console.log(`Reviewer returned. stdout follows:\n${reviewResult.stdout}`);