Preserve manual handoff notes from SessionEnd overwrite

[jonschull]

Summary

• Fixes SessionEnd hook overwrites manual handoff notes #566
• When the CTO writes a detailed handoff mid-session, the SessionEnd hook was overwriting it with Haiku's lossy summary
• Now checks if the handoff file was modified within the last 30 minutes — if so, appends Haiku's summary as an addendum instead of replacing

Test plan

• Write a manual handoff note, then trigger SessionEnd — verify manual content preserved with Haiku addendum appended
• Wait >30 min (or touch with old mtime), trigger SessionEnd — verify normal overwrite behavior unchanged

🤖 Generated with Claude Code

[claude-ERA]

Auto-merged

PR #581 merged and deployed.

Smoke test: WARNING — see output

Verify: https://era-website.onrender.com

Test output

Traceback (most recent call last):
  File "/Users/admin/era2c/era2/website/tests/smoke_test.py", line 12, in <module>
    from playwright.sync_api import sync_playwright
ModuleNotFoundError: No module named 'playwright'