fix(thread-ledger): restore the os import the ledger split dropped

`main` resolves the default turn-summary path through `os.homedir()`,
and the entry point's rewritten import block did not carry `node:os`.
Every `ledger declare` without TURN_SUMMARY_PATH set threw
`ReferenceError: os is not defined`.

The suite never saw it: the declare tests exercise `declareText`, which
is pure, so the CLI branch that builds the fallback path had no test at
all. It has one now — the CLI run as a process, with HOME pointed at a
temp directory and the env var unset, asserting the file lands at
`$HOME/.claude/turn-summary.txt`. It fails on the broken import and
passes on the fix.

ADVANCES #188

Claude-Session: https://claude.ai/code/session_014CUXJh1hKmW4ccUwRQ1Ep1

original/thread-ledger/ledger.mjs

@@ -19,6 +19,7 @@
 // Node builtins only. Pushes with plain git; no forge API, no MCP.
 
 import fs from "node:fs";
+import os from "node:os";
 import path from "node:path";
 import process from "node:process";
 import { fileURLToPath } from "node:url";

tests/original/thread-ledger/test_cli.mjs

@@ -360,6 +360,29 @@ describe("declare", () => {
     }
   });
 
+  it("writes to the default path when nothing names one", () => {
+    // The CLI's own resolution, run as a process: `declareText` is pure
+    // and never reaches it, so the branch that builds the fallback path
+    // had no test and a missing import broke it silently (#188).
+    const dir = fs.mkdtempSync(path.join(os.tmpdir(), "declare-home-"));
+    try {
+      const result = spawnSync(
+        process.execPath,
+        [path.join(SKILL, "ledger.mjs"), "declare", "--reviews", "none"],
+        {
+          encoding: "utf8",
+          env: { ...process.env, HOME: dir, TURN_SUMMARY_PATH: undefined },
+        },
+      );
+      assert.equal(result.status, 0, result.stderr);
+      const written = path.join(dir, ".claude", "turn-summary.txt");
+      assert.ok(fs.existsSync(written), `nothing at ${written}`);
+      assert.match(fs.readFileSync(written, "utf8"), /^reviews: none$/m);
+    } finally {
+      fs.rmSync(dir, { recursive: true, force: true });
+    }
+  });
+
   it("writes the two core lines even when empty", () => {
     const text = declareText({ reviews: "none" });
     assert.equal(text, "tickets: \nreviews: none\n");