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/env.local.zsh.example

@@ -0,0 +1,4 @@
+# Copy to ~/.config/zsh/local/env.local.zsh (installer never overwrites local/).
+# Runs for every zsh after managed env modules.
+#
+# . "$HOME/.cargo/env"