Add `--since` filter to `eval-harness trend` (e.g. --since=7d, --since=2026-05-01)

[hoainho]

Why

eval-harness trend reads the full history.ndjson and renders all of it. On a project with 6 months of history, that's noise. People want "show me the last 7 days" or "show me since the v0.4.0 release date."

What to build

Add --since=<spec> to scripts/eval/trend.sh. Accept:

• Nd / Nw / Nm (e.g. 7d, 2w, 1m) → compute the cutoff relative to now
• YYYY-MM-DD (ISO date) → absolute cutoff
• <git-ref> (e.g. v0.4.0, HEAD~50) → resolve via git log -1 --format=%ct <ref> to a timestamp

Filter history.ndjson lines by timestamp >= cutoff.

Acceptance criteria

• trend.sh accepts and parses --since per the spec above
• Invalid spec → exits 64 with a helpful error
• tests/trend_since.sh covers all 3 spec formats + invalid input
• README "Trend" section documents the new flag

Good first issue because

• Single file (trend.sh)
• Logic is "parse a string, compare a timestamp" — no LLM, no spawn, no lock
• ~60 lines of bash + 1 test
• The date-parsing branch is a fun puzzle (portable across GNU/BSD date)

Comment to claim.

[muhammadaaribali]

Hi! I'd like to work on this issue.