fix(acr): pin Bash >> append discipline on shared implementation log (M-BUG-20)

implement.md spawns implementer agents in parallel batches, all appending to
the same implementation-log.md. Dogfooding showed agents satisfying 'Append
result to:' with a full-file Write — the last writer clobbered 4 of 6 entries.
Pin the mechanism in both contracts: append with Bash >> heredoc, never the
Write/Edit tool on the shared log. Shape tests pin the instruction in both
files (empirically verified: agents given the >> instruction appended safely).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/commands/implement-log-append.test.mjs

@@ -0,0 +1,41 @@
+/**
+ * M-BUG-20 — shared implementation-log clobbering under parallel agents.
+ *
+ * implement.md step 4 spawns implementer agents in parallel batches, and every
+ * agent appends its result to the SAME implementation-log.md. Dogfooding
+ * (2026-07-17, throwaway linkedin-posts copy) showed agents satisfying
+ * "Append result to:" with a full-file Write: each agent read the log, added
+ * its entry, and wrote the whole file back — the last writer silently
+ * clobbered 4 of 6 entries.
+ *
+ * Contract: both the command template and the agent prompt must pin the append
+ * mechanism — Bash `>>`, never the Write/Edit tool — on the shared log.
+ */
+
+import { test } from 'node:test';
+import { strict as assert } from 'node:assert';
+import { readFile } from 'node:fs/promises';
+import { resolve, dirname } from 'node:path';
+import { fileURLToPath } from 'node:url';
+
+const __dirname = dirname(fileURLToPath(import.meta.url));
+const ROOT = resolve(__dirname, '..', '..');
+
+const APPEND_MECHANISM_REGEX = />>/;
+const FORBID_WRITE_TOOL_REGEX = /never[^.\n]*\bwrite\b[^.\n]*tool|\bwrite\b[^.\n]*tool[^.\n]*never/i;
+
+test('implement.md: agent-spawn template pins Bash >> append on the shared log', async () => {
+  const content = await readFile(resolve(ROOT, 'commands', 'implement.md'), 'utf-8');
+  assert.match(content, APPEND_MECHANISM_REGEX,
+    'implement.md must instruct appending to implementation-log.md with Bash >>');
+  assert.match(content, FORBID_WRITE_TOOL_REGEX,
+    'implement.md must forbid the Write tool on the shared implementation log');
+});
+
+test('implementer-agent.md: output section pins Bash >> append and forbids Write tool on the log', async () => {
+  const content = await readFile(resolve(ROOT, 'agents', 'implementer-agent.md'), 'utf-8');
+  assert.match(content, APPEND_MECHANISM_REGEX,
+    'implementer-agent.md must instruct appending to the log with Bash >>');
+  assert.match(content, FORBID_WRITE_TOOL_REGEX,
+    'implementer-agent.md must forbid the Write tool on the shared implementation log');
+});