docs: condense comments across the repo

Comments had drifted into multi-paragraph narrative (git commit
lineage, debugging stories, restated code) in several hot spots
(scripts/deploy, hermes-agent.nix, flake.nix, gitea.nix, headscale.nix).
Trim every comment to its load-bearing "why" — gotchas, safety
warnings, and non-obvious rationale survive verbatim in substance,
just tightened to 1-2 sentences; historical narrative and anything
already covered in CLAUDE.md is cut. No code/logic changed.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UJqEmY1y3AYX3JoX4Y6b21

home/common.nix

@@ -8,10 +8,9 @@
   programs.home-manager.enable = true;
 
   # Matches terra's baseline (compinit, deduped/shared history, HISTFILE
-  # under $HOME). home-manager owns ~/.zshrc + ~/.zshenv as real files, which
-  # also means zsh's built-in zsh-newuser-install wizard never fires on
-  # first interactive login (it only triggers when none of
-  # .zshenv/.zprofile/.zshrc/.zlogin exist) — that used to happen on every
-  # host except terra.
+  # under $HOME). home-manager owning ~/.zshrc + ~/.zshenv as real files also
+  # means zsh's newuser-install wizard never fires (it only triggers when
+  # none of those dotfiles exist) — previously an issue on every host except
+  # terra.
   programs.zsh.enable = true;
 }