0.17.0 — Explore-Before-Review & Base Branch Awareness

The reviewer now runs the full explore pipeline before generating a review, giving the review agent richer codebase context. PR events also carry the target branch for better diff orientation.

What's new

• Explore-before-review — when using ApiAgentConfig and no pre-existing context is supplied, the reviewer automatically runs the explore pipeline (planner + parallel explorers) before the review turn. Explorer and planner provider configs fall back to the reviewer config when not explicitly set. Exploration failures are caught and logged; the review continues without context.
• Base branch on PR events — PullRequestEvent now carries a base_branch field, populated from GitHub (base.ref) and GitLab (target_branch) webhook payloads and round-tripped through job serialization. The reviewer prompt includes the base branch when available.
• Explore metrics on ReviewResult — ReviewResult.explore_metrics exposes the aggregated exploration metrics (turns, API calls, tokens, group count) so callers can track exploration cost alongside review cost.
• Provider lifecycle — _run_explore_for_review creates and closes its own LLM provider, keeping resource management contained within the exploration step.