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.

Dockerfile.variant

@@ -190,11 +190,13 @@ ARG USER_NAME=developer
 # (published 2026-10-01T19:15Z) and neither obsmem nor atelier has a commit
 # naming it. Acceptance must prove the obsmem workers CAP TURNS (peerDeps are
 # `*`, so a mismatch is silent) and that the atelier sidebar PAINTS.
-# USER-VISIBLE BEHAVIOUR CHANGE, deliberately NOT overridden here: 1.0.0 makes
-# the TUI fullscreen by default, which replaces 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.
+# USER-VISIBLE BEHAVIOUR CHANGE, decided rather than inherited: 1.0.0 makes the
+# TUI fullscreen by default, which replaces the terminal's normal scrollback.
+# Upstream's default is ADOPTED on purpose and no `tuiMode` is baked here, so
+# the image follows pi instead of pinning the fleet to either mode. The revert
+# is documented (README -> "Terminal UI mode"): `"tuiMode": "regular"` in
+# ~/.pi/agent/settings.json, `pi --tui-mode regular` for one session, or the
+# same key in a project's .pi/settings.json.
 ARG PI_VERSION=1.0.0
 ARG PI_TOOLKIT_REF=main
 ARG PI_EXTENSIONS_REF=main