refactor(shell): thin stubs with managed ~/.config/zsh tree

Split always-on PATH/env, login hooks, and interactive UX into a
managed tree under ~/.config/zsh, with never-overwrite local/ hooks.
Agents and interactive shells share one PATH owner; deploy.sh applies
the layout safely across machines.

local/interactive.local.zsh.example

@@ -0,0 +1,2 @@
+# Copy to ~/.config/zsh/local/interactive.local.zsh (installer never overwrites local/).
+# Interactive only, after oh-my-zsh and managed post-omz aliases.