fix(Dockerfile): declare PI_VERSION in base stage (ARG is stage-scoped)

The de-fork commit replaced the fork clone with 'npm install -g
@earendil-works/pi-coding-agent@${PI_VERSION}' but left the base stage's
ARG redeclaration list with the dead PI_FORK/PI_REF entries and no
PI_VERSION. ARGs are stage-scoped, so the ${PI_VERSION} in the RUN was
passed to the shell unsubstituted and 'set -u' aborted the image build
('PI_VERSION: parameter not set') — caught by the 0.0.74-lpb-dev CI run.

Replace the dead ARG PI_FORK/PI_REF with ARG PI_VERSION=0.84.4 (default
keeps local builds without --build-arg working, matching the global ARG).

Author: localpibox

Dockerfile

@@ -52,8 +52,7 @@ FROM ubuntu:26.04 AS base
 
 ARG NODE_VERSION
 ARG VSCODIUM_VERSION
-ARG PI_FORK
-ARG PI_REF
+ARG PI_VERSION=0.84.4
 ARG PI_HEAD_SHA
 ARG CONFIG_FORK
 ARG CONFIG_REF