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

common.nix

@@ -54,10 +54,9 @@
   environment.systemPackages = with pkgs; [ git btop tmux curl wget zsh-powerlevel10k lsd jq ];
 
   # ---- home-manager (user-level config for darman, all hosts) ----
-  # Requires home-manager.nixosModules.home-manager in the host's own
-  # `modules` list (flake.nix) — this only sets values for options that
-  # module declares, it doesn't import it, so every nixosSystem using
-  # common.nix needs that line too (mirrors terra's original setup).
+  # Only sets values for options declared by home-manager.nixosModules.home-manager;
+  # it doesn't import that module, so every nixosSystem using common.nix must
+  # also list it in flake.nix's `modules`.
   home-manager.useGlobalPkgs = true;
   home-manager.useUserPackages = true;
   # Protects activation if a plain (non-symlink) ~/.zshrc etc. already
@@ -91,10 +90,9 @@
   boot.loader.systemd-boot.configurationLimit = 5;
   boot.loader.generic-extlinux-compatible.configurationLimit = 5;
 
-  # Stock journald defaults to ~10% of the filesystem (up to 4G) before it
-  # rotates — no scheduled vacuum, just a ceiling it grows into. On jupiter's
-  # 29G eMMC that's ~2.9G it could silently accumulate. Cap it well below that
-  # everywhere instead of only noticing when a disk fills up again.
+  # Stock journald grows unbounded up to ~10% of the filesystem (4G cap, no
+  # scheduled vacuum) — on jupiter's 29G eMMC that's ~2.9G it could silently
+  # fill. Cap it well below that everywhere.
   services.journald.extraConfig = ''
     SystemMaxUse=200M
   '';