docs(v1.10.0): adopt pi's fullscreen default and document the way back

Decision: keep pi 1.0.0's default (tuiMode "fullscreen"). No tuiMode is baked,
so the image follows upstream rather than pinning the fleet to either mode -
but "we inherited a changed default" is only acceptable if the revert is
written down, so it now is.

README gains a "Terminal UI mode" section ahead of the pi-atelier section,
because the two interact. It gives all three scopes, taken from pi 1.0.0's own
docs/settings.md and docs/cli.md rather than from the changelog prose:

  - permanent: "tuiMode": "regular" in ~/.pi/agent/settings.json
  - one session: pi --tui-mode regular   (the flag is real: cli.md:221)
  - one project: the same key in .pi/settings.json, which overrides the agent
    directory

It also documents the four related settings (fullscreenExitOutput,
fullscreenScrollbar, fullscreenCopyOnSelect, fullscreenWheelScrollLines) and
two things specific to this image:

  - tmux: fullscreen uses the alternate screen, so tmux copy-mode shows the
    pane history AROUND pi, not pi's transcript. That is the concrete reason
    someone here would want "regular" back.
  - fullscreenWheelScrollLines "auto" behaves differently over SSH (caps fast
    wheel spins at 6 lines/event) than in a local macOS terminal (1 line) -
    worth naming because this container is normally driven over SSH.
  - pi-atelier works in BOTH modes (upstream has handled regular vs fullscreen
    renderers separately since pi 0.84), so reverting costs nothing. Stated so
    nobody assumes the sidebar is the price of the old scrollback.

The python3 merge snippet in that section was RUN before being documented:
against a realistic settings.json under an overridden HOME, twice, confirming
it is idempotent and preserves sibling keys including nested objects and the
_comment fields the seeded file uses. Documented code that has never been
executed is a guess.

DOCKER_HUB.md gets a short version of the same note, because CI reads it from
the TAG and POSTs it to Docker Hub as full_description - a behaviour change
this visible should not require reading the repo to undo.

Gates: doc-drift 23 OK / 0 DRIFT / 0 SKIP / 0 FAIL; base-hash, workflow-shell,
skill-floor, lint-shell all rc=0.

CHANGELOG.md

@@ -111,12 +111,19 @@ jump, four new base packages and a new mailbox feature. The release carrying a
   are `*`, so a mismatch is silent at runtime) and that the atelier sidebar
   **paints**.
 
-  **User-visible behaviour change, deliberately not overridden.** 1.0.0 makes
-  the TUI fullscreen by default, replacing the terminal's normal scrollback;
-  `tuiMode: "regular"` restores the old behaviour. No baked default is set, so
-  the image inherits upstream's choice rather than silently pinning the fleet to
-  either one. If the fleet wants the old scrollback, that is a one-line settings
-  change, not a rebuild.
+  **User-visible behaviour change: the TUI is now fullscreen.** 1.0.0 changed the
+  default `tuiMode` to `"fullscreen"`, which draws into the terminal's alternate
+  screen — so the transcript no longer accumulates in the terminal's (or tmux's)
+  native scrollback. **Upstream's default is adopted deliberately**, not
+  inherited by omission: no `tuiMode` is baked, so the image follows pi rather
+  than pinning the fleet to either mode. The way back is documented in README
+  → *Terminal UI mode*: `"tuiMode": "regular"` in `~/.pi/agent/settings.json`
+  for every session, `pi --tui-mode regular` for one, or the same key in a
+  project's `.pi/settings.json` for one project. That section also covers the
+  four related `fullscreen*` settings — including `fullscreenWheelScrollLines`,
+  whose `"auto"` behaviour differs over SSH, which is how this container is
+  normally driven — and records that pi-atelier's sidebar works in both modes,
+  so reverting costs nothing.
 
 - **pi-atelier `v0.10.3` (`ed3837b`) → `v0.13.0` (`34d26f1`)** — closes the
   two-minor gap v1.9.5 named as the pi bump's residual risk. `peerDependencies`