chore(claude): add SessionStart compact context injector

inject-sprint-context.sh fires on SessionStart with matcher=compact
and emits branch, last 10 commits, and the top of BACKLOG.md so
Claude resumes with sprint context after auto-compaction. Output
capped at ~600 tokens.

.claude/hooks/inject-sprint-context.sh

@@ -0,0 +1,29 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+cd "$CLAUDE_PROJECT_DIR" 2>/dev/null || exit 0
+
+branch="$(git branch --show-current 2>/dev/null || echo unknown)"
+recent="$(git log --oneline -10 2>/dev/null || echo '(git log unavailable)')"
+
+backlog_excerpt="(BACKLOG.md not found)"
+if [ -f dev-docs/BACKLOG.md ]; then
+  # First 40 lines, capped at ~3500 chars to stay well under 600 tokens for the whole output.
+  backlog_excerpt="$(head -n 40 dev-docs/BACKLOG.md | head -c 3500)"
+fi
+
+cat <<EOF
+## Sprint context (re-injected after compaction)
+
+**Branch:** $branch
+
+**Last 10 commits:**
+$recent
+
+**Top of BACKLOG.md:**
+$backlog_excerpt
+
+Reminder: re-read /CLAUDE.md if any zero-compromise principle is unclear. /dev-docs/SCHEMA.md is authoritative for table structure.
+EOF
+
+exit 0