fix(signal): evict oldest sent timestamp instead of arbitrary one

[dieutx]

Summary

• _track_sent_timestamp() uses set.pop() to evict when the echo-back filter exceeds 50 entries
• set.pop() removes an arbitrary element, not the oldest — could drop a recent timestamp while keeping old ones
• This causes the echo-back filter to miss actual echo-backs in Note to Self / self-chat mode
• Fix: use min() to always evict the oldest timestamp

How to reproduce

• Use Signal Note to Self / self-chat mode
• Send 50+ messages rapidly
• Some echo-backs may slip through the filter because recent timestamps get evicted randomly

How to test

• All 33 signal tests pass
• The fix is a 1-line change: set.pop() → set.discard(min(set))

Platform tested

• Linux, hermes-agent v0.4.0

[dieutx]

Closing — small fix with no tests, cleaning up stale PRs.