feat(cli): agentv history read command for per-test score timeline

[christso]

Part of #1155.

Objective

Add a read-only agentv history subcommand that surfaces per-test score timelines from a sidecar baseline file by parsing its git log. Gives users "continuous improvement tracking" of eval scores over time without requiring any new storage format or server.

Why

AgentV's sidecar <eval>.baseline.jsonl files are already committed to repos; each time someone runs agentv compare --update-baseline (see #1158), the file gets a new commit. git log -p on that file is already the source of truth for how scores have evolved. agentv history is a thin CLI wrapper that renders that log as a per-test timeline.

Design latitude

# Timeline for one test across all commits that touched the baseline file:
agentv history --baseline evals/my-eval.baseline.jsonl --test "handles empty input"

# Example output:
# commit       date        score  verdict
# abc123def    2026-04-23  0.82   pass
# def456abc    2026-04-22  0.79   borderline
# ...

# JSONL for piping into downstream tools:
agentv history --baseline evals/my-eval.baseline.jsonl --test "handles empty input" --format jsonl

• Uses git plumbing (git log --follow -p <file>) to walk the baseline's history.
• Parses each revision of the JSONL to extract the named test's score and verdict.
• --since <date> and --until <date> for range filtering (optional).
• No new storage, no server, no config.

Acceptance signals

• Works on a repo where the baseline file has at least 3 commits touching it (can be set up with a small test fixture).
• Outputs a correctly-ordered (newest first) timeline.
• --format jsonl pipes cleanly into jq.
• Errors helpfully if the baseline file isn't tracked in git, or if the named test doesn't appear in any revision.

Non-goals

• Not a dashboard. Terminal/JSONL output only.
• Not a cross-repo or cross-file aggregator — one baseline file per invocation.
• Not writing to or mutating git history.

Depends on

Nothing new — works against AgentV's existing sidecar baseline convention. More useful once #1158 ships (because --update-baseline will produce regular commits to parse), but does not require it.