entrypoint: link fleet pi extensions from the mounted config dir

pi auto-loads every .ts in ~/.pi/agent/extensions/, which lives in the
devbox-pi-config volume. A baked extension only seeds a fresh volume once, and a
hand-copied one is a per-device step that drifts — the same two problems
link-pi-models.sh was added for on 2026-10-12, one level down. This is also the
only path that reaches every host: four of them mount common/pi from
docker-compose-repo, and three cannot be reached from a devbox to create a
symlink by hand (measured: no ssh config entry, no name resolution, and this
instance's ~/.ssh-local/config carries no jump alias — only gitea and the
mempalace feed host are reachable).

link-pi-extensions.sh links each TOP-LEVEL *.ts from the mounted fleet directory
(default /opt/fleet-pi, the same directory that already carries models.json;
PI_EXTENSIONS_SRC overrides, =off disables). A symlink, not a single-file bind
mount, for the measured reason already documented for models.json: git pull
replaces a file by rename, and a file mount then serves stale content. Never
destructive — a real file of that name in ~/.pi/agent/extensions/ wins over the
fleet copy. Nested *.ts are deliberately not linked: the directory is fleet pi
config, not an extension tree.

- scripts/test-link-pi-extensions.sh: 16 two-sided cases (the negative half is
  the contract), all passing, wired into lint.yml beside the models one.
- smoke-test.sh and recreate-sanity-check.sh gained the matching in-image and
  on-device checks, each gated on the fleet directory actually holding *.ts so a
  device without a shared directory is not judged by them.
- README: the shared directory can carry extensions, and the rules that apply.

Also repairs a pre-existing release blocker, found while checking this change.
check-doc-drift.sh failed on pristine HEAD (verified in a clean worktree):
pi-extensions main moved 0d8e417 -> 4c1d28d without the SHA being named above
the v1.11.2 heading, and docker-publish.yml's resolve-versions has
needs: [lint-gate] — so the next tag could not have published. The CHANGELOG now
names 4c1d28d and describes the two confirm-destructive commits from their diff
(they stop the gate prompting on harmless redirects, and add tee/dd of= as write
forms); node --test test/confirm-destructive.test.mjs at 4c1d28d: 2 pass, 0 fail.

Local verification: lint-shell.sh (21 files, severity error), check-workflow-shell.sh,
check-skill-floor.sh, check-dockerfile-directives.sh, check-doc-drift.sh, and both
link-pi-*.sh contract suites all pass. actionlint and hadolint are not installed in
a devbox, so those CI steps are unvalidated here; the smoke-test addition runs only
inside the image at release time.

scripts/smoke-test.sh

@@ -675,6 +675,7 @@ exec_test "mempalace.ts bridge"                    'test -L $HOME/.pi/agent/exte
 exec_test "settings.json bootstrapped"             'test -f $HOME/.pi/agent/settings.json && echo ok'
 exec_test "no shared models.json mounted → none linked" 'test ! -e $HOME/.pi/agent/models.json && test ! -L $HOME/.pi/agent/models.json'
 exec_test "link-pi-models.sh links a mounted models.json" 'mkdir -p /tmp/lpm && echo "{}" > /tmp/lpm/models.json && PI_MODELS_JSON=/tmp/lpm/models.json PI_CODING_AGENT_DIR=/tmp/lpm/agent bash /usr/local/lib/pi-devbox/link-pi-models.sh >/dev/null && test "$(readlink /tmp/lpm/agent/models.json)" = /tmp/lpm/models.json && rm -rf /tmp/lpm'
+exec_test "link-pi-extensions.sh links a mounted *.ts" 'mkdir -p /tmp/lpe/src && echo "export default {}" > /tmp/lpe/src/x.ts && PI_EXTENSIONS_SRC=/tmp/lpe/src PI_CODING_AGENT_DIR=/tmp/lpe/agent bash /usr/local/lib/pi-devbox/link-pi-extensions.sh >/dev/null && test "$(readlink /tmp/lpe/agent/extensions/x.ts)" = /tmp/lpe/src/x.ts && rm -rf /tmp/lpe'
 exec_test "pi-devbox-environment skill linked"     'test -L $HOME/.agents/skills/pi-devbox-environment && test -f $HOME/.agents/skills/pi-devbox-environment/SKILL.md && echo ok'
 exec_test "pi-extensions skill linked (fallback)"  'test -L $HOME/.agents/skills/pi-extensions && test -f $HOME/.agents/skills/pi-extensions/SKILL.md && echo ok'
 exec_test "mempalace skill linked (fallback)"      'test -L $HOME/.agents/skills/mempalace && test -f $HOME/.agents/skills/mempalace/SKILL.md && echo ok'