Add guardrail for repeated edit_file failures

Tracks 'old_str not found' errors from edit_file tool calls. After 5
consecutive failures, stops the agent to prevent burning tokens on
impossible edits.

This catches the pattern where the agent repeatedly tries to edit a
large file with incorrect old_str matches, which was the root cause
of t-222 exceeding its cost budget.

Task-Id: t-224

0 files changed, 0 insertions, 0 deletions