fix(zim): skip Zim update when Zim is not configured

[hlclarog]

Closes #285

Problem

scripts/package/update_all runs under set -u and calls zsh "$ZIM_HOME/zimfw.zsh" unconditionally. When ZIM_HOME is not set, the step aborts with ZIM_HOME: unbound variable, and the run still reports that every package was updated.

Fix

Move the Zim update into scripts/package/src/package_managers/zim.sh, like the other package managers, and guard it the same way the other steps are guarded:

• zim::is_configured is true only when ZIM_HOME is set and $ZIM_HOME/zimfw.zsh exists.
• zim::update_all keeps the existing upgrade and update calls and the log::file output unchanged.
• update_all runs zim::is_configured && output::h2 "👽 Zim Framework" && zim::update_all, so the step is skipped silently when Zim is not configured.

Testing

• With stubbed zsh, log::file and output::answer, under set -euo pipefail:
  • ZIM_HOME unset: zim::is_configured is false and nothing aborts (before: unbound variable);
  • ZIM_HOME set without zimfw.zsh: zim::is_configured is false;
  • ZIM_HOME set with zimfw.zsh: zim::update_all runs zimfw.zsh upgrade then zimfw.zsh update and prints Done, the same as before.
• scripts/self/static_analysis and scripts/self/lint pass.