Allow overriding the Codex zsh launcher

[germanKoch]

Summary

• Allow the Codex Desktop Zsh launcher path to be overridden with CODEX_OBSIDIAN_ZSH_PATH.
• Preserve /usr/local/bin/zsh as the default for backward compatibility.
• Apply the override consistently across all six CLI domain skills and their playbooks.
• Document the environment variable and add it to the validation checklist.

Why

The sanitized macOS launcher currently hardcodes /usr/local/bin/zsh. That path is not present on every Mac, so otherwise valid Obsidian CLI commands fail before Zsh starts. The root cause is the fixed executable path rather than Obsidian CLI availability.

Impact

Existing users see no behavior change when the variable is unset. Users whose Zsh lives elsewhere can set an absolute path, for example CODEX_OBSIDIAN_ZSH_PATH=/bin/zsh, without editing the plugin.

Validation

• git diff --check
• Parsed .codex-plugin/plugin.json and .agents/plugins/marketplace.json with jq.
• Confirmed the unset expression resolves to /usr/local/bin/zsh.
• Ran obsidian version successfully with CODEX_OBSIDIAN_ZSH_PATH=/bin/zsh.
• Ran read-only smoke tests for vault, bookmarks, runtime-admin, devtools, Sync, and workspace command families through the overridden wrapper.