Add JSON and HTML reporters for eval results

[Cryptoteep]

Context

The CLI promptkit eval command prints a plain-text table to stdout. That's fine for local use, but in CI you often want a machine-readable artifact to upload, trend over time, or render in a PR check.

Proposal

Add a --reporter flag to promptkit eval:

promptkit eval ./evals/sentiment.eval.ts --reporter json --out results.json
promptkit eval ./evals/sentiment.eval.ts --reporter html --out results.html

Reporters:

• text (default, current behaviour)
• json — full EvalResult serialised, plus a small summary header
• html — a self-contained static HTML page with a per-case table

Implementation notes

• Create src/cli/reporters/ with text.ts, json.ts, html.ts.
• Define a Reporter interface: report(result: EvalResult): string.
• The HTML reporter should be a single string template — no external CSS/JS deps. Keep it ~100 lines.
• Add a --out flag that writes to a file instead of stdout (text reporter still goes to stdout by default).

Acceptance

• --reporter and --out flags wired into the eval command.
• All three reporters implemented + unit-tested.
• docs/cli.md updated with examples.
• CI uploads the JSON artifact as a smoke test (optional).

/label "good first issue" "help wanted" "area:cli" "area:eval" "enhancement"