fix(panes): /tui works in panes with workspace instructions

Claude refused `/tui` ("Cannot switch renderers in this session — it has
restrictions a restart can't carry over") in every pane started with
--append-system-prompt-file, i.e. any workspace with instructions (member
panes usually have the owner's). The instructions are now written as the only
CLAUDE.md of a folder of their own (<data dir>/workspace-prompts/<id>/, a
member's prompt-<id>/ in the handoff dir via write-as), passed as one more
--add-dir with CLAUDE_CODE_ADDITIONAL_DIRECTORIES_CLAUDE_MD=1. Verified with
Claude Code 2.1.301: the file is listed in /memory and /tui switches. Codex
is unchanged (its pointer line names the same file).

server/__tests__/server-s3-handoff.test.js

@@ -114,7 +114,7 @@ describe("a member pane's files live in the member's handoff dir", () => {
     assert.deepEqual(x.addDirs, [ctx]);
     assert.equal(fs.readFileSync(path.join(ctx, "spec.md"), "utf8"), "# spec\n");
     assert.equal(mode(path.join(ctx, "spec.md")), 0o644);
-    assert.equal(x.systemPromptFile, path.join(HANDOFF, `prompt-${laneId}.md`));
+    assert.equal(x.systemPromptFile, path.join(HANDOFF, `prompt-${laneId}`, "CLAUDE.md"));
     assert.equal(mode(x.systemPromptFile), 0o644);
     const text = fs.readFileSync(x.systemPromptFile, "utf8");
     assert.match(text, /Be brief\./);