Add Gentoo support with GURU overlay requirement

Tadbit (tadbit-gentoo, x86_64, gcc 15.2.1, TKG kernel) joins the
homelab as the 7th machine — the first gentoo box. Adds Gentoo
support to the bootstrap alongside arch and debian.

New scripts:
- run_once_10-add-gentoo-overlays.sh.tmpl: enables GURU overlay
  via 'eselect repository enable guru' + 'emaint sync -r guru'.
  Idempotent — skips if GURU is already at /var/db/repos/guru.
  GURU is required because eza, lazygit, topgrade, and most modern
  CLI tools only live in GURU (not main).
- run_once_20-install-user-packages-gentoo.sh.tmpl: emerge-based
  user package set. Writes USE flags to package.use/ BEFORE
  emerging so foot/wofi/waybar get the right features. Detects
  missing packages via 'command -v <basename>'. Falls back to
  the official curl installer for bun (no gentoo package).
- run_once_40-install-sway-gentoo.sh.tmpl: sway stack via emerge
  with USE flags for X+wayland+tray+upower+wireplumber.

Updated scripts:
- .chezmoi.yaml.tmpl: os_family detection now also matches 'gentoo'.
  Critical fix: Gentoo's /etc/os-release uses single-quoted values
  ('gentoo' not 'gentoo' or "gentoo"), and chezmoi's parser doesn't
  strip them. Without trimAll "'", .chezmoi.osRelease.id returns
  the literal string 'gentoo' with quotes, and the eq test fails.
  Symptom: os_family silently becomes 'unknown'.
- run_once_00-install-bootstrap-tools.sh.tmpl: added gentoo branch
  that uses emerge --sync + emerge (skipping if tree is < 1 day old).
- run_onchange_30-ensure-cargo.sh.tmpl: added gentoo branch for
  bat (already installed by emerge, just verify), topgrade (GURU),
  cargo-update (dev-util/cargo-update in main).

README: documented Gentoo-specific quirks (USE flags, GURU,
single-quote parsing, no binary packages).

.chezmoi.yaml.tmpl

@@ -3,10 +3,19 @@
 {{- if hasKey .chezmoi.osRelease "idLike" -}}
   {{- $idLike = .chezmoi.osRelease.idLike -}}
 {{- end -}}
-{{- if or (eq .chezmoi.osRelease.id "arch") (contains "arch" $idLike) -}}
+{{- $osId := .chezmoi.osRelease.id -}}
+{{- /* Gentoo's /etc/os-release uses single-quoted values (e.g. ID='gentoo'),
+     and the parser doesn't strip them, so strip here. Other distros use
+     double-quoted or unquoted values which the parser handles. */ -}}
+{{- if hasPrefix "'" $osId -}}
+  {{- $osId = trimAll "'" $osId -}}
+{{- end -}}
+{{- if or (eq $osId "arch") (contains "arch" $idLike) -}}
   {{- $osFamily = "arch" -}}
-{{- else if or (eq .chezmoi.osRelease.id "debian") (contains "debian" $idLike) -}}
+{{- else if or (eq $osId "debian") (contains "debian" $idLike) -}}
   {{- $osFamily = "debian" -}}
+{{- else if eq $osId "gentoo" -}}
+  {{- $osFamily = "gentoo" -}}
 {{- end -}}
 encryption: "age"
 # chezmoi's builtin age implementation requires a TTY for passphrase-style
@@ -71,7 +80,7 @@ age:
 
 data:
     os_family: {{ $osFamily | quote }}
-    os_id: {{ .chezmoi.osRelease.id | quote }}
+    os_id: {{ $osId | quote }}
 
     # Sway/Wayland desktop stack — opt-in at bootstrap time. When you
     # run `chezmoi init` for the first time, you'll be asked: