feat(signals): act on typed report links in research, auto-start and completion

docs/internal/signals-pr-lifecycle.md

@@ -8,6 +8,20 @@ The shared PR-linking service applies this rule to task outputs and agent attach
 An existing attachment retry does not reopen a report, and importing legacy assignments preserves its status.
 Suppressed reports remain suppressed when another PR is attached.
 
+A report that is `part_of` another report is a step in a plan, and the plan completes from its steps.
+When every live step of a plan is closed, the plan takes their verdict: resolved if at least one step resolved, suppressed if they all were.
+A plan with any step still open is left alone, and a deleted step counts neither way.
+Reopening a step does not reopen a completed plan.
+Deleting a step also checks the plan again.
+A plan that becomes ready checks its own steps, in case they closed while the plan was still in research.
+The check locks each plan before it reads and updates its state.
+It continues through an already closed plan to check that plan's parent.
+An archived step never undoes a resolved plan.
+The roll-up runs on the step's own status change, so a merged PR, a manual resolve, a bulk state change, and an MCP state write all reach it.
+A `part_of` link written on a step that already closed runs the check as well, because that write changes no status.
+It continues up a plan of plans, and skips a plan that is waiting on a replacement.
+It also skips a plan that carries its own open, draft, or unknown PR, because that plan's own work decides its status.
+
 ## Repository selection
 
 The shared repository selection prompt asks the agent to check the sources in the supplied context before choosing a repository.
@@ -28,12 +42,35 @@ If no suggested reviewer has access, the ready report still goes to the configur
 Only scouts and the signals pipeline create and manage typed, directed report links.
 Scouts attach them through the `links` list on `scout-edit-report`.
 Public callers can read `report_link` artefacts, but cannot create, edit, or delete them through the artefact API.
+Typed links cannot yet be removed through a supported operation.
+[Issue #102776](https://github.com/PostHog/posthog/issues/102776) tracks the unlink API and MCP tool.
+The Implement button overrides an automatic start check; it does not remove a link or change plan membership.
 Links must name a different live report in the same project and cannot form a cycle among links of the same kind.
 
+Research reads a report's outgoing `follow_up_of`, `depends_on`, and `part_of` links and starts from the linked reports' findings and pull requests.
+A linked report must have an explicit safe verdict in its latest safety judgment.
+Research uses the latest finding for each signal and removes repeated links.
+It loads at most ten distinct relationships it can use and limits the rendered context to 12,000 characters.
+All linked fields are escaped and marked as untrusted evidence.
+A follow-up can refer to a manual fix or a regression; it must not invent a missing pull request.
+
+Three link gates hold back automatic implementation, and each one records why on the report:
+
+- A report that duplicates another one does not start its own work when any report on that duplicate chain is resolved or already carries a pull request.
+- A report that depends on another one does not start until that dependency carries a pull request.
+- A report that other reports are `part_of` never starts its own work, because its steps do the work.
+
+Link readers use the writer database so a new link takes effect without replica delay.
+The implementation path checks the gates again under the report lock before it creates a task.
+The gates apply to automatic implementation only.
+Pressing Implement in the inbox starts a run whatever the links say.
+Nothing re-evaluates a held-back report when its dependency's pull request opens, so it starts on the report's next pipeline evaluation or by hand.
+
 ## Recurrence after a fixed verdict
 
 A report dismissed as `already_fixed`, `fixed_outside_posthog`, or `pr_merged` can create a new report when the issue returns.
 The pipeline records the new report's parent with a typed `recurrence_of` report link.
+A report created because a follow-up check failed on a resolved report also gets a typed `follow_up_of` link to that report, carrying the verdict as the link's reason.
 Generic `related_to` links do not control signal assignment.
 Later signals follow the recurrence chain, even if an older parent is restored.
 Matching selects the current successor before the specificity check, so the check uses its signals and title.

frontend/snapshots.yml

@@ -11636,6 +11636,10 @@ snapshots:
         hash: v1.k794b7964.4d8cc63186f867316fa053a9a1c26a29141d9a8519fc0431f9ee8f246fb87535.nM5FGnojjy0SQYBwxGuPDW_CkV-B77jh96snsiQDHrQ
     scenes-app-signals-artefactloglist--check-lifecycle-narrow--light:
         hash: v1.k794b7964.323182dc2e4da1cc3de88a75d4d027f1d9b561a74752a6e9efda62c307252811.Z7e7sMgVVYTvNNOumDp3_1YmGPVrwuL3MtLQZITadIQ
+    scenes-app-signals-artefactloglist--linked-report-gates--dark:
+        hash: v1.k794b7964.62d15835041cea269d599d42d6f654f2e9ed4ae1e39c3f3b5eddd5833fca6394.AcjuW_1a3hPiT3q-Bn7FtXcctGRSm87IsWJU0NTgPK4
+    scenes-app-signals-artefactloglist--linked-report-gates--light:
+        hash: v1.k794b7964.859a100531c36c4b874e64d2bd0b2500476c8b922f8cdeee1462d1d5d6804506.R61Z2ikhPeSQuqB9dx5_VB6HF-IAjCuhVo932jK6Rt8
     scenes-app-signals-artefactloglist--narrow--dark:
         hash: v1.k794b7964.16c8190934d879b7eb46d67ee86637e34003d27e1503dfe0f52cd403f8b7ea0c.zw0vIR4b3_skhadCSU_SQkInLL9Bn0oyVSAXcZddxJo
     scenes-app-signals-artefactloglist--narrow--light:

posthog/test/repo_invariants/setup_receivers_baseline.txt

@@ -221,6 +221,7 @@ post_save:products.signals.backend.receivers.close_pr_when_report_dismissed
 post_save:products.signals.backend.receivers.emit_report_embedding_on_document_change
 post_save:products.signals.backend.receivers.mark_scout_suggestions_stale_on_fleet_change
 post_save:products.signals.backend.receivers.reconcile_report_embedding_on_verdict_saved
+post_save:products.signals.backend.receivers.roll_up_plan_parents_when_report_closes
 post_save:products.signals.backend.receivers.schedule_handover_for_report_change
 post_save:products.signals.backend.receivers.schedule_handover_for_work_change
 post_save:products.signals.backend.receivers.schedule_implementation_handover