fix: support fork PR intent comments

[z2z23n0]

Mainline Intent

Intent: int_807c5590
Status: proposed
Title: 为 Mainline PR Intent Action 增加 fork PR 支持

What changed

将 Mainline PR Intent workflow 迁移为单一 pull_request_target 路径：同仓 PR 继续 checkout 并执行 head 代码，fork PR 只 checkout 可信 base，并把 PR head 作为 Git 数据读取。pr-comment 新增可选的 fork URL/PR 参数，可从 fork actor refs 临时投影 contributor-published intents，复用原有 commit-range、多 intent、branch fallback、marker 和 sticky comment 行为。加入并发取消、head 新鲜度校验、live PR body 检查和临时 ref 清理，并补充 engine/CLI 回归测试。

Why

普通 pull_request 在 fork PR 上只有只读 token，无法写评论；直接移除同仓限制又会让高权限 job 执行不可信 fork 代码。新的可信 base 执行路径在不改变现有同仓评论能力的前提下安全读取 fork intent metadata，并避免在 review 阶段导入或修改 upstream Mainline 状态。

Decisions

• GitHub Actions 触发与 checkout 模型: 使用单一 pull_request_target workflow；同仓 checkout head，fork checkout 精确 base SHA。 (保持一个稳定的 Mainline PR Intent check，避免 fork 同时出现旧 Skipped 和新 check；fork 路径始终只运行 default/base 上的可信代码。)
  • Rejected: 保留 pull_request 再新增 fork-only workflow，因为 GitHub 无法在 on 层按 head repo 过滤，会产生重复或 Skipped checks
• fork intent metadata 的信任边界: 只把 fork actor refs fetch 到临时 imports refs 并物化只读 view；upstream canonical intent 同 ID 时始终优先。 (PR comment 是 review surface，不是 actor-log acceptance；预览不能调用 ImportActorLog、写 canonical actor refs/notes/view 或 push。)
  • Rejected: 在 PR 打开阶段复用 pr-import，因为它会接受 contributor metadata 并可能更新 upstream refs/notes
• fork 与同仓评论语义: 复用现有 commit-range 优先、多 intent、branch fallback、排序和 Markdown renderer，并给未接受的 fork intent 增加 provenance 提示。 (reviewer 获得与同仓 PR 一致的 intent 内容，同时能区分 contributor-published metadata 尚未被 upstream 接受。)
• 并发与过期事件处理: 按 PR 号取消旧 run，并在生成前后校验 live head/body。 (防止 synchronize/edited 的旧事件覆盖新评论；PR body 已有 Mainline marker 时直接清理旧 sticky comment 并成功退出。)

Subsystems: .github, cli, engine, github-actions, pull-request, intent-rendering, fork-actor-log

[Copilot]

Pull request overview

This PR updates Mainline’s PR intent comment pipeline to safely support fork-based pull requests by shifting the GitHub Actions workflow to pull_request_target and adding an engine/CLI pathway that reads fork-published intent metadata as Git data without importing it into upstream state.

Changes:

• Added PRCommentWithOptions and CLI flags (--pr, --fork-url) to render fork actor-log intent metadata in PR comments without syncing/importing.
• Introduced fork intent discovery + rendering (including provenance messaging) and added targeted engine/CLI tests for fork behavior.
• Migrated the Mainline PR Intent workflow to a single pull_request_target flow with concurrency cancellation and stale-head checks.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
internal/engine/pr.go	Adds PullRequestCommentOptions + fork-aware PR comment rendering entrypoint.
internal/engine/pr_fork.go	Implements fork actor-log discovery/fetch and merges fork intents into PR comment view.
internal/engine/pr_fork_test.go	Adds regression tests ensuring fork intent preview doesn’t import/accept upstream state and filters terminal intents.
internal/cli/pr.go	Wires new pr-comment options (--pr, --fork-url) to the engine options struct.
internal/cli/root_test.go	Ensures new pr-comment flags are registered.
.github/workflows/mainline-pr-intent.yml	Switches to pull_request_target, adds concurrency, stale event checks, and fork-safe checkout/fetch strategy.

Comments suppressed due to low confidence (3)

internal/engine/pr_fork.go:121

• Similarly, decoding failures for specific fork event payloads (sealed/abandoned/superseded) currently abort the entire PR comment generation. Since these payloads are also untrusted, treat decode failures as a skip for that event instead of failing the run.

		case domain.EventIntentSealed:
			var event domain.IntentSealedEvent
			if err := json.Unmarshal(raw, &event); err != nil {
				return nil, domain.NewError(
					domain.ErrInvalidInput,
					fmt.Sprintf("fork sealed event %s is invalid: %v", base.EventID, err),
				)
			}

internal/engine/pr_fork.go:133

• For fork PR previews, an invalid abandoned event payload should not fail the entire PR intent comment generation (untrusted fork input). Skip malformed abandoned events instead.

		case domain.EventIntentAbandoned:
			var event domain.IntentAbandonedEvent
			if err := json.Unmarshal(raw, &event); err != nil {
				return nil, domain.NewError(
					domain.ErrInvalidInput,
					fmt.Sprintf("fork abandoned event %s is invalid: %v", base.EventID, err),
				)
			}

internal/engine/pr_fork.go:145

• For fork PR previews, an invalid superseded event payload should not fail the entire PR intent comment generation (untrusted fork input). Skip malformed superseded events instead.

		case domain.EventIntentSuperseded:
			var event domain.IntentSupersededEvent
			if err := json.Unmarshal(raw, &event); err != nil {
				return nil, domain.NewError(
					domain.ErrInvalidInput,
					fmt.Sprintf("fork superseded event %s is invalid: %v", base.EventID, err),
				)
			}