Add self-contained runtime bundle distribution (jlink) for hosts without Java

./gradlew runtimeBundle packs a jlink-trimmed JRE, gittally.jar, and a
launcher script into one tarball, unpacked to ~/opt/gittally on the target
host; init --systemd works from the bundle unchanged because java.home and
the running-jar path resolve into it. Chosen over a GraalVM native image
(Spring AOT evaluates bean conditions at build time, which cannot represent
the dual-context CLI/server wiring) and over a containerized runtime — see
ADR 0006 and docs/plan/15-runtime-bundle-distribution.md, which also records
the full vm2176-to-vm4006 migration walkthrough.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

packaging/gittally

@@ -0,0 +1,6 @@
+#!/bin/sh
+# Launcher for the self-contained GitTally runtime bundle (jlink JRE + jar).
+# Built by `./gradlew runtimeBundle`; see docs/deployment.md.
+DIR=$(CDPATH='' cd -- "$(dirname -- "$(readlink -f -- "$0")")" && pwd)
+# shellcheck disable=SC2086 # JAVA_OPTS is intentionally word-split
+exec "$DIR/../jre/bin/java" $JAVA_OPTS -jar "$DIR/../lib/gittally.jar" "$@"