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

hosts/mercury/secrets.nix

@@ -2,11 +2,10 @@
 
 # sops-nix wiring for mercury. Encrypted values in ../../secrets/mercury.yaml.
 #
-# SD images have no `--extra-files` step, so mercury uses a DEDICATED age key
-# placed on the ROOT filesystem (the Pi's vfat partition isn't mounted at
-# runtime — u-boot reads it pre-boot). `./deploy flash mercury <dev>` drops
-# ~/.config/homelab/mercury/age.txt there automatically.
-# The key never enters the repo, the nix store, or the image itself.
+# SD images get no `--extra-files` step, so mercury uses a dedicated age key
+# on the root filesystem instead of the admin key — the Pi's vfat boot
+# partition isn't mounted at runtime (u-boot reads it pre-boot), so the key
+# can't live there.
 {
   sops.defaultSopsFile = ../../secrets/mercury.yaml;
   sops.age.keyFile = "/var/lib/sops-nix/age.txt";