feat: persist & restore per-project input mode (#14)

The input mode always started in simple; a learner who quit in advanced
had to re-toggle every launch. Store the mode per-project in project.yaml
(project.mode:, optional, default simple) and restore it on every open.

Mode is live UI state, not schema: the worker stamps the current mode
into project.yaml on every write, so a later command rewrites the live
value rather than clobbering it — no db round-trip needed. The mode is
persisted on unload (quit + project switch) so the mode you leave a
project in is always what reopens; the `mode` command also persists
immediately. A switch saves the outgoing mode, then restores the
incoming project's stored mode.

New --mode simple|advanced CLI flag (precedence --mode > stored >
simple; combines with --resume). A teacher can ship a project that
opens in advanced mode and export it to students (the mode travels in
the zip).

ADR-0015 Amendment 1; ADR-0003 note; help banner; requirements L1b.

docs/requirements.md

@@ -598,6 +598,16 @@ since ADR-0027.)
   exclusive with a positional path argument (ADR-0015 §7).
   `last_project` is rewritten on every successful project
   open (startup, load, new, save as, import).
+- [x] **L1b** Per-project input-mode restore (issue #14,
+  ADR-0015 Amendment 1). The input mode is stored in
+  `project.yaml` (`project.mode:`), restored on every open, and
+  persisted on a `mode` change and on unload (quit / project
+  switch) — so the mode you leave a project in is what reopens.
+  A teacher can ship a project that opens in advanced mode; a
+  learner's last-used mode is restored per project. New
+  `--mode simple|advanced` CLI flag (precedence `--mode` >
+  stored > `simple`; combines with `--resume`). Independent of
+  the `history.log` input-history hydration piece of Iteration 6.
 - [~] **L2** Submit a command alongside project load — deferred,
   not v1.