fix: use shared scratch helpers in dev-agent and action-agent to eliminate duplicates (#262)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

action/action-agent.sh

@@ -24,6 +24,7 @@ export PROJECT_TOML="${2:-${PROJECT_TOML:-}}"
 
 source "$(dirname "$0")/../lib/env.sh"
 source "$(dirname "$0")/../lib/agent-session.sh"
+source "$(dirname "$0")/../lib/formula-session.sh"
 # shellcheck source=../dev/phase-handler.sh
 source "$(dirname "$0")/../dev/phase-handler.sh"
 SESSION_NAME="action-${ISSUE}"
@@ -168,22 +169,8 @@ if [ -n "${_thread_id:-}" ]; then
 fi
 
 # --- Read scratch file (compaction survival) ---
-SCRATCH_CONTEXT=""
-if [ -f "$SCRATCH_FILE" ]; then
-  SCRATCH_CONTEXT="## Previous context (from scratch file)
-$(cat "$SCRATCH_FILE")
-"
-fi
-SCRATCH_INSTRUCTION="## Context scratch file (compaction survival)
-
-Periodically (every 10-15 tool calls), write a summary of:
-- What you have discovered so far
-- Decisions made and why
-- What remains to do
-to: ${SCRATCH_FILE}
-
-If you find this file exists when you start, read it first — it is your previous context.
-This file is ephemeral — not evidence or permanent memory, just a compaction survival mechanism."
+SCRATCH_CONTEXT=$(read_scratch_context "$SCRATCH_FILE")
+SCRATCH_INSTRUCTION=$(build_scratch_instruction "$SCRATCH_FILE")
 
 # --- Build initial prompt ---
 PRIOR_SECTION=""