feat(retention): run-record pruning - keep newest N, dry-run default (#32)

- mosaic-task.mjs prune [--keep=N] [--yes]: default keep 50; without
  --yes lists candidates without deleting
- only r-* directories under the runs root; symlinks skipped;
  sessions/workspaces/state/config untouched (asserted by suite sentinels)
- append-only receipt runs/.pruned.log records every pruned id
- test-task.sh: +8 retention cases (dry-run no-delete, keep-N, newest
  kept, receipt, isolation, invalid keep, empty no-op)

Also: suite hardening - prune section scopes its config per-command
(no export/unset leaking into later sections); duplicated check()
removed; latest_reason hoisted to helpers; status colors now green OK /
red FAIL (terminal-only, NO_COLOR-aware) per owner UX feedback.

Closes #32

scripts/mosaic-task.mjs

@@ -529,6 +529,52 @@ function retryRun(runId) {
   runTask(tempTaskFile, { retriedFrom: runId });
 }
 
+function pruneRuns(args) {
+  const resolved = loadConfig();
+  const root = runsRoot(resolved);
+  let keep = 50;
+  let apply = false;
+  for (const arg of args) {
+    if (arg === "--yes") apply = true;
+    else if (arg === "--keep") fail(4, "prune: --keep requires a value");
+    else if (arg.startsWith("--keep=")) {
+      keep = Number(arg.slice("--keep=".length));
+      if (!Number.isInteger(keep) || keep < 1) fail(4, `--keep must be a positive integer (got ${arg.slice(7)})`);
+    } else fail(4, `unknown prune option: ${arg}`);
+  }
+
+  let entries = [];
+  try {
+    entries = fs.readdirSync(root, { withFileTypes: true })
+      .filter((e) => e.name.startsWith("r-") && e.isDirectory() && !e.isSymbolicLink())
+      .map((e) => e.name)
+      .sort();
+  } catch {
+    // No runs yet.
+  }
+
+  if (entries.length <= keep) {
+    process.stdout.write(`prune: ${entries.length} run(s) present, keep=${keep} -> nothing to prune\n`);
+    process.exit(0);
+  }
+
+  const doomed = entries.slice(0, entries.length - keep); // oldest first
+  if (!apply) {
+    process.stdout.write(`prune (dry-run): would remove ${doomed.length} oldest run(s), keep ${entries.length - doomed.length}:\n`);
+    for (const id of doomed) process.stdout.write(`  would remove: ${id}\n`);
+    process.stdout.write("prune: re-run with --yes to apply\n");
+    process.exit(0);
+  }
+
+  const receipt = path.join(root, ".pruned.log");
+  for (const id of doomed) {
+    fs.rmSync(path.join(root, id), { recursive: true, force: true });
+    fs.appendFileSync(receipt, `${JSON.stringify({ at: new Date().toISOString(), event: "pruned", runId: id })}\n`);
+  }
+  process.stdout.write(`prune: removed ${doomed.length} run(s), kept ${keep}; receipt: ${receipt}\n`);
+  process.exit(0);
+}
+
 const operation = process.argv[2];
 const target = process.argv[3];
 
@@ -552,10 +598,13 @@ switch (operation) {
   case "list":
     listRuns();
     process.exit(0);
+  case "prune":
+    pruneRuns(process.argv.slice(3));
+    break;
   case "retry":
     if (!target) fail(4, "usage: mosaic-task.mjs retry <runId>");
     retryRun(target);
     break;
   default:
-    fail(4, `unknown operation: ${JSON.stringify(operation ?? "")} (expected validate | run | show | list | retry)`);
+    fail(4, `unknown operation: ${JSON.stringify(operation ?? "")} (expected validate | run | show | list | retry | prune)`);
 }