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, 14 insertions, 0 deletions

diff --git a/cli/src/core/errors.ts b/cli/src/core/errors.ts
new file mode 100644
index 0000000..0bab4bd
--- /dev/null
+++ b/cli/src/core/errors.ts
@@ -0,0 +1,14 @@
+/**
+ * An error the user can act on, as opposed to a bug.
+ *
+ * These are printed and the process exits non-zero. They deliberately do NOT
+ * trigger the fail-open handover to sl0ppij: "no project called foo" or "that
+ * session is a saved dead copy" are answers, and replacing them with a raw
+ * multiplexer would hide the answer and start a session nobody asked for.
+ */
+export class UserError extends Error {
+  constructor(message: string) {
+    super(message);
+    this.name = "UserError";
+  }
+}