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).

run_once_10-add-gentoo-overlays.sh.tmpl

@@ -0,0 +1,56 @@
+#!/usr/bin/env bash
+# =============================================================================
+#  run_once_05-add-gentoo-overlays.sh.tmpl  (gentoo-only)
+#  Add the GURU overlay to the portage repository list. GURU is the
+#  community overlay (like AUR for arch) where packages like bun, eza,
+#  sway, etc. live when they're not in the main tree.
+#
+#  Idempotent: skips if GURU is already enabled.
+#  Run-order: 00 (bootstrap) → 05-overlays (here) → 10-add-{chaotic,debian}
+#  → 20-install-packages → 40-install-sway
+# =============================================================================
+set -euo pipefail
+
+log() { printf '\033[1;34m[overlays]\033[0m %s\n' "$*"; }
+die() { printf '\033[1;31m[overlays ERROR]\033[0m %s\n' "$*" >&2; exit 1; }
+
+# This script is gentoo-only. Guarded by the chezmoi template engine;
+# on arch/debian it never renders (only this single file path).
+
+if [[ ! -d /var/db/repos/gentoo ]]; then
+    die "/var/db/repos/gentoo missing — this doesn't look like a gentoo system"
+fi
+
+# --- 1. GURU overlay ---
+# User explicitly requires GURU. Per the bootstrap-runbook skill: gentoo
+# boxes without GURU can't install most of the user packages (eza, fzf,
+# fd-find on stable, lazygit, topgrade, etc.) because they only ship in
+# the main tree as ~amd64 or only live in GURU.
+if [[ -d /var/db/repos/guru ]]; then
+    log "GURU overlay already enabled at /var/db/repos/guru — skipping"
+else
+    log "enabling GURU overlay via eselect repository"
+    # `eselect repository` needs the overlays.xml index; sync first if missing
+    if ! sudo eselect repository list 2>&1 | grep -qi guru; then
+        log "fetching overlays.xml index"
+        sudo emaint sync --auto
+    fi
+    sudo eselect repository enable guru
+    # Sync just the new repo so we don't re-pull the entire gentoo tree
+    log "syncing GURU overlay"
+    sudo emaint sync -r guru
+    log "GURU overlay enabled and synced"
+fi
+
+# --- 2. Verify ---
+log "enabled overlays:"
+eselect repository list 2>&1 | sed 's/^/  /'
+
+# Verify the GURU repo is readable
+if [[ ! -f /var/db/repos/guru/profiles/repo_name ]]; then
+    die "/var/db/repos/guru/profiles/repo_name missing — overlay didn't sync cleanly"
+fi
+GURU_NAME=$(cat /var/db/repos/guru/profiles/repo_name)
+log "GURU repo verified: $GURU_NAME"
+
+log "overlays ready"