[codex] Follow up ranking readability suggestions

[coe0718]

What Changed

This is a tiny follow-up to the optional readability suggestions on #2.

• replaces the 3-input weightedAverage(..., 0, ..., 0) call in calculateEpisodeContextScore() with a dedicated 2-input helper
• simplifies hoursSince() to use Date.parse() directly instead of a redundant Number.isNaN wrapper
• adds a focused regression test to confirm invalid timestamps still degrade gracefully

Why

PR #2 was merged as-is, but the review included two non-blocking suggestions to make the ranking helper a little easier to read. This keeps that cleanup separate from the original memory feature work.

How I Tested

bash -lc 'export PATH="$HOME/.bun/bin:$PATH" && /home/coemedia/.bun/bin/bun test src/memory/__tests__/ranking.test.ts'
/home/coemedia/.bun/bin/bun run typecheck
/home/coemedia/.bun/bin/bun run lint
bash -lc 'export PATH="$HOME/.bun/bin:$PATH" && /home/coemedia/.bun/bin/bun test'