Give UI-queued investigation tasks the same identity as auto-filed ones (isInvestigation marker + fingerprint)

[atomantic]

The installer-failure "Queue agent to investigate" button (#5981) queues a CoS task with { useWorktree: true, openPR: true }, but the task carries no investigation identity, so it sits outside the machinery that governs every auto-filed investigation.

Gap

server/lib/investigationTasks.js defines what an investigation task IS:

• INVESTIGATION_TASK_DELIVERY = { useWorktree, openPR, prCompletion: MERGE_ON_GREEN } — the UI-queued task matches the first two and omits the third.
• isInvestigationTask() keys on metadata.isInvestigation (with a legacy headline-prefix fallback). A UI-queued task matches neither.
• buildInvestigationFingerprint() is the durable dedup key consumed by the reaper (cosTaskStore.js) and the retry decision (services/investigationRetry.js).

Consequences for a UI-queued investigation:

1. It is invisible to the investigation dedup scan and the circuit breaker, so nothing bounds how many pile up beyond the generic same-description 409.
2. It is not covered by the meta-cascade guard — if the queued run itself fails, agentErrorAnalysis can file an auto-investigation of the investigation.
3. It does not participate in investigation auto-retry.

createCosTaskSchema (server/lib/cosValidation.js) has no isInvestigation field, so Zod strips any client attempt to set one — this cannot be fixed client-side.

Scope

• Decide the identity a client-queued investigation gets. Recommended: add a narrowly-typed isInvestigation boolean (or a small investigation: { source, fingerprint } object) to createCosTaskSchema, routed by cosTaskStore.addTask into metadata.isInvestigation exactly as the server-side producer does — the client supplies the flag, never a hand-crafted fingerprint.
• Derive the fingerprint server-side from the submitted context so a client cannot collide with, or evict, an auto-filed investigation.
• Have QueueInstallInvestigationButton (client/src/components/install/QueueInstallInvestigationButton.jsx) set the flag, and reconcile its INSTALL_INVESTIGATION_DELIVERY constant with INVESTIGATION_TASK_DELIVERY — including whether a user-initiated install investigation should also carry prCompletion: merge-on-green (a UI-queued repair is user-initiated, so review-then-merge may be the better default; make the call and document it).
• Cover the round-trip: a UI-queued task is recognized by isInvestigationTask(), is deduped against a matching auto-filed one, and does not trigger a meta-cascade when it fails.

Acceptance criteria

• A task queued from the installer-failure button is recognized by isInvestigationTask().
• It participates in investigation dedup / the circuit breaker, and a failure of it does not cascade into an auto-filed investigation of itself.
• The delivery posture divergence is resolved (matched to INVESTIGATION_TASK_DELIVERY, or deliberately different with the reason recorded in code).
• Tests cover the schema field, the store routing, and the recognition/dedup path.

Related to #5981.