feat(discord): compact plain-text notifications with bug fixes (#1021)

- Fix esc() bracket expression (] must be first in POSIX regex)
- Fix delete job: inline helper to avoid checkout of deleted ref
- Fix issue notifications: attribute close/reopen to actor, not author
- Simplify trunc() comment (remove false Unicode-safe claim)
- Smart truncation with wall-of-text detection
- Escape markdown and @mentions for safe display

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: Brian <bmadcode@gmail.com>

.github/scripts/discord-helpers.sh

@@ -0,0 +1,15 @@
+#!/bin/bash
+# Discord notification helper functions
+
+# Escape markdown special chars and @mentions for safe Discord display
+# Bracket expression: ] must be first, then other chars. In POSIX bracket expr, \ is literal.
+esc() { sed -e 's/[][\*_()~`>]/\\&/g' -e 's/@/@ /g'; }
+
+# Truncate to $1 chars (or 80 if wall-of-text with <3 spaces)
+trunc() {
+  local max=$1
+  local txt=$(tr '\n\r' '  ' | cut -c1-"$max")
+  local spaces=$(printf '%s' "$txt" | tr -cd ' ' | wc -c)
+  [ "$spaces" -lt 3 ] && [ ${#txt} -gt 80 ] && txt=$(printf '%s' "$txt" | cut -c1-80)
+  printf '%s' "$txt"
+}