cli: survive a dead session, and a flaky terminal

Three bugs found on a fresh Debian box, all of which made sl0ppi look broken
when the terminal underneath it was the problem.

1. JSON crash on startup. `action list-panes --json` against a session that has
   died exits 0 with EMPTY stdout, and JSON.parse("") threw "JSON Parse error:
   Unexpected EOF" at the user. Empty output is now "nothing", and genuinely
   malformed output is reported with the command name and the offending text.

2. `up` silently resurrected. A dead-but-resurrectable session is not an empty
   slot: `attach --create-background NAME` restores it from the serialised copy
   and IGNORES --layout. The result was a root full of projects coming back as
   plain shells with no agents and no purposes -- and the resurrected background
   session then exited again within seconds. sl0ppi now refuses to guess and
   explains the two real options (`up --fresh` to rebuild, `sl0ppij attach` to
   restore it properly in a terminal). Verified on a clean slate that a fresh
   `up` produces exactly the right panes: purpose='agent:main' held=true cmd='pi'.

3. The terminal's CLI actions are racy. Measured against a LIVE session on a
   2-vCPU box: list-tabs and list-panes each fail ~60% of the time while the
   session is detached, ~25% with a client attached, either with empty output or
   "There is no active session!" (the action client consults
   get_active_session() before it looks at -s). That is a defect in sl0ppij --
   filed as a known issue -- but these are idempotent reads, so sl0ppi retries
   them with backoff and distinguishes "the session is gone" from "the terminal
   would not answer". `sl0ppi ls` went from failing most runs to 8/8.

Also:
  - session creation returns before the server is registered, so `up` now waits
    for readiness instead of racing its own next command
  - UserError: actionable answers ("no project foo", "that session is a saved
    dead copy") print and exit 1 instead of falling through to sl0ppij, which
    buried the explanation and started a session nobody asked for
  - `sl0ppi print <project>` implemented; it was referenced by the init template
    and the design doc but did not exist

1 files changed, 39 insertions, 0 deletions

diff --git a/cli/test/sessions.test.ts b/cli/test/sessions.test.ts
new file mode 100644
index 0000000..00e55a2
--- /dev/null
+++ b/cli/test/sessions.test.ts
@@ -0,0 +1,39 @@
+import { describe, expect, test } from "bun:test";
+import { parseJsonList } from "../src/core/sl0ppij.ts";
+
+// The crash a user actually hit: `action list-panes --json` on a session that
+// died underneath us exits 0 with EMPTY stdout, and JSON.parse("") threw
+// "JSON Parse error: Unexpected EOF" at them on startup.
+describe("parseJsonList", () => {
+  test("empty output is nothing, not a crash", () => {
+    expect(parseJsonList("", "list-panes")).toEqual([]);
+    expect(parseJsonList("   \n", "list-panes")).toEqual([]);
+  });
+
+  test("parses a normal listing", () => {
+    expect(parseJsonList('[{"position":0}]', "list-tabs")).toEqual([{ position: 0 }] as never);
+  });
+
+  test("genuinely malformed output names the command and shows the text", () => {
+    expect(() => parseJsonList("There is no active session!", "list-tabs")).toThrow(
+      /list-tabs returned something that is not JSON: There is no active session!/,
+    );
+  });
+
+  test("a non-array payload is treated as nothing rather than crashing later", () => {
+    expect(parseJsonList('{"error":"nope"}', "list-tabs")).toEqual([]);
+  });
+});
+
+import { UserError } from "../src/core/errors.ts";
+
+// Actionable answers must not be replaced by a raw multiplexer. Falling through
+// on "no project called foo" hides the answer AND starts a session nobody asked
+// for.
+describe("UserError", () => {
+  test("is distinguishable from a bug", () => {
+    expect(new UserError("nope") instanceof UserError).toBe(true);
+    expect(new UserError("nope") instanceof Error).toBe(true);
+    expect(new Error("boom") instanceof UserError).toBe(false);
+  });
+});