feat: add --json and --since flags to jilog review nightly

[mvanhorn]

Summary

jilog review nightly --json and jilog review nightly --dry-run --since 24h now run as documented in the README.

Why this matters

The README's Quick Start shows jilog review nightly --json | your-agent synthesize-suggestions (line 52) and the dry-run example shows jilog review nightly --dry-run --since 24h (line 57), but NightlyArgs only exposes --days, --digest-dir, --dry-run, --create-issues, --date, --processed-file. Anyone copy-pasting from the README hits an unknown-flag error.

This PR closes that drift in code rather than walking the README back, so the documented agent-synthesis pipe works end to end.

Changes

• --json (boolean): emit a single JSON object to stdout and suppress the human summary lines so the output is one parseable document. The query subcommand already accepts --format json, so the JSON path is consistent with existing precedent inside the same binary.
• --since <duration>: accepts Nh, Nd, Nw, or YYYY-MM-DD. Reuses pub(crate) fn parse_since from commands::query. parse_since is extended with an hour suffix so --since 24h from the README example works as written.
• --days is retained for back-compat. --since and --days conflict only when both are user-supplied (clap v4 evaluates conflicts_with via ValueSource::CommandLine, so --since 24h alone with the default --days 1 does not raise).
• DigestReport is left non-Serialize. The JSON output is hand-built as a serde_json::Value inside commands/review.rs, so this PR stays in the jilog crate.

JSON shape

{
  "schema_version": 1,
  "sessions_scanned": 0,
  "corrections": 0,
  "errors": 0,
  "workarounds": 0,
  "p0_alerts": { "<tool_name>": ["<session_id>", ...] },
  "digest_path": "/abs/path/learning-digest-2026-05-10.md",
  "created_issues": [{"id": "...", "backend": "...", "title": "...", "url": "..."}]
}

schema_version: 1 reserves room for additive evolution. p0_alerts is the structured tool-to-sessions map (not a count) so a downstream agent can synthesize from the JSON alone without re-parsing the markdown digest. created_issues carries the IssueRef fields the markdown digest does not expose. digest_path is null under --dry-run. Happy to adjust the shape if you had something different in mind for v1.

Testing

• cargo build --workspace passes
• cargo test --workspace passes
• Smoke-tested jilog review nightly --dry-run --since 24h and jilog review nightly --dry-run --json | jq .
• 6 new unit tests cover: --since alone with default --days does not conflict, --since + user --days does conflict, --since 24h matches --days 1 window within 2s, --since parser error path, JSON output keys present, --json --dry-run emits digest_path: null.

[Joi]

Hi Mvan — thanks for the first PR! Both of these are excellent. Reading through them is a pleasure: sharp motivation, careful scoping, tests that exercise the subtle behavior (the clap conflicts_with quirk here, the workaround/deferral overlap in #2).

This one I'm merging as-is. schema_version: 1 is exactly the right move, p0_alerts as a structured map (not a count) is the right design, and the reuse of parse_since keeps the parser unified. One micro-thought for a future PR if you ever want to revisit: serde_json::to_string_pretty is friendly for inspection but jq/JSONL pipelines tend to prefer compact one-line output. Not a blocker for this one.

Thanks again — looking forward to more.

— Joi