chore(installer): shellcheck-clean the archiso answerfile flow

shellcheck passes (default severity) on the base installer and the whole
answerfile chain: generate-answerfile.sh → build.sh → .automated_script.sh →
launch.sh → arch-autoinstall.sh → tui-install.sh. Verified the generator's
JSON schema (drive, kernel, keymap, hostname, username, encrypt, fido2_root,
fido2_user, run_tui, components, desktop_environment, apps, shell_rc, colors)
parses correctly through both the auto base installer (af_get/af_bool) and the
TUI installer (load_answerfile), for full and minimal/empty answerfiles.

Fixed the three legitimate findings surfaced along the way:
- generate-answerfile.sh: drop unused AVAIL_DRIVES (SC2034); the drive list is
  rendered inline in the dialog prompt.
- tui-install.sh: drop unused C_RULE (SC2034); write log truncation as
  ': > "$LOG"' so the redirection has an explicit command (SC2188).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

setup/generate-answerfile.sh

@@ -101,8 +101,8 @@ dialog --backtitle "$BACKTITLE" \
 # ═══════════════════════════════════════════════════════════════
 
 # ── Drive ─────────────────────────────────────────────────────────────────────
-AVAIL_DRIVES=$(lsblk -dn -o NAME,SIZE,MODEL 2>/dev/null | awk '{printf "%s \"%s %s\" off ", $1, $2, $3}' || true)
-
+# The available-drives list is rendered inline in the inputbox prompt below via
+# lsblk, so no separate variable is needed here.
 AF_DRIVE=$(dialog --backtitle "$BACKTITLE" \
     --title " Target Drive " \
     --inputbox "\n  Enter the install drive (e.g. /dev/sda, /dev/nvme0n1).\n\n  Available drives:\n$(lsblk -dn -o NAME,SIZE,MODEL 2>/dev/null | sed 's/^/    /')\n" \

setup/tui-install.sh

@@ -37,7 +37,6 @@ ANSWERFILE_MODE=false
 # tput is not used here because it may be unavailable on a minimal TTY.
 C_RESET=$'\033[0m'
 C_TITLE=$'\033[1;35m'   # bold magenta — section headers
-C_RULE=$'\033[35m'      # magenta — separators
 C_ACCENT=$'\033[36m'    # cyan — prompts and selected marks
 C_BOLD=$'\033[1m'
 C_DIM=$'\033[2m'
@@ -461,7 +460,8 @@ fi
 
 # Truncate the log file now that preflight passed; all prior output was to stdout.
 # Entries appended from this point on are the authoritative install log.
-> "$LOG"
+# ':' is the no-op command so the redirection has an explicit target.
+: > "$LOG"
 printf "Dotfiles install: %s\nDotfiles dir:    %s\n" "$(date)" "$DOTFILES_DIR" >> "$LOG"
 
 # ── Welcome ───────────────────────────────────────────────────────────────────