migrate: stow → chezmoi

- restructure repo to chezmoi conventions (dot_ prefix, private_dot_ssh/)
- add nvim config (was untracked)
- encrypt restic pass.txt with age
- remove stow artifacts (.stow-local-ignore, stow package dirs)
- update bootstrap.sh to use chezmoi apply
- remove runtime files from tracking (micro buffers, zed backup)

dot_config/nvim/lazyvim.json

@@ -0,0 +1,11 @@
+{
+  "extras": [
+    "lazyvim.plugins.extras.ai.copilot",
+    "lazyvim.plugins.extras.lang.json"
+  ],
+  "install_version": 8,
+  "news": {
+    "NEWS.md": "11866"
+  },
+  "version": 8
+}