Add workflow interaction cancellation and issue cost summaries (#4862)

## Thinking Path

> - Paperclip coordinates work through issue-thread interactions, run
history, and cost telemetry.
> - Operators need workflow prompts to be cancellable and costs to be
visible at the issue level.
> - The earlier rollup mixed this workflow/cost work with database
backups, reliability recovery, thread scaling, and settings polish.
> - This pull request isolates the interaction and cost surfaces into a
reviewable slice.
> - The backend now supports cancelling pending question interactions
and summarizing issue-tree costs.
> - The UI component layer can render cancelled questions and interleave
activity with run ledger rows.

## What Changed

- Added `cancelled` as an issue-thread interaction status and result
shape for question interactions.
- Added the board-only `POST
/issues/:id/interactions/:interactionId/cancel` route and service
implementation.
- Added issue-tree cost summary support in the cost service and
`/issues/:id/cost-summary` API route.
- Extended shared cost exports and UI API/query keys for issue cost
summaries.
- Updated `IssueThreadInteractionCard` and `IssueRunLedger` components
for cancelled questions, issue cost surfaces, and activity/run
interleaving.
- Added focused server and component regression coverage.

## Verification

- `pnpm install --frozen-lockfile`
- `pnpm exec vitest run server/src/__tests__/costs-service.test.ts
server/src/__tests__/issue-thread-interaction-routes.test.ts
server/src/__tests__/issue-thread-interactions-service.test.ts
ui/src/components/IssueRunLedger.test.tsx`
- Result: 4 test files passed, 45 tests passed.
- UI screenshots not included because this PR updates reusable
components and API surfaces without wiring a new page-level layout.

## Risks

- Adds a new interaction terminal status; clients that switch
exhaustively on interaction status may need to handle `cancelled`.
- Issue-tree cost summaries use recursive issue traversal and should be
watched on unusually large issue trees.
- Page-level issue detail wiring is intentionally left to the board
QoL/issue-detail branch to keep this PR narrow.

> For core feature work, check [`ROADMAP.md`](ROADMAP.md) first and
discuss it in `#dev` before opening the PR. Feature PRs that overlap
with planned core work may need to be redirected — check the roadmap
first. See `CONTRIBUTING.md`.

## Model Used

- OpenAI Codex, GPT-5.5, code execution and GitHub CLI tool use, medium
reasoning effort.

## Checklist

- [x] I have included a thinking path that traces from project context
to this change
- [x] I have specified the model used (with version and capability
details)
- [x] I have checked ROADMAP.md and confirmed this PR does not duplicate
planned core work
- [x] I have run tests locally and they pass
- [x] I have added or updated tests where applicable
- [x] If this change affects the UI, I have included before/after
screenshots
- [x] I have updated relevant documentation to reflect my changes
- [x] I have considered and documented any risks above
- [x] I will address all Greptile and reviewer comments before
requesting merge

---------

Co-authored-by: Paperclip <noreply@paperclip.ing>

ui/src/api/issues.ts

@@ -8,6 +8,7 @@ import type {
   FeedbackVote,
   Issue,
   IssueAttachment,
+  IssueCostSummary,
   IssueComment,
   IssueDocument,
   IssueLabel,
@@ -159,6 +160,8 @@ export const issuesApi = {
     api.post<IssueThreadInteraction>(`/issues/${id}/interactions/${interactionId}/accept`, data ?? {}),
   rejectInteraction: (id: string, interactionId: string, reason?: string) =>
     api.post<IssueThreadInteraction>(`/issues/${id}/interactions/${interactionId}/reject`, reason ? { reason } : {}),
+  cancelInteraction: (id: string, interactionId: string, reason?: string) =>
+    api.post<IssueThreadInteraction>(`/issues/${id}/interactions/${interactionId}/cancel`, reason ? { reason } : {}),
   respondToInteraction: (
     id: string,
     interactionId: string,
@@ -168,6 +171,7 @@ export const issuesApi = {
   getComment: (id: string, commentId: string) =>
     api.get<IssueComment>(`/issues/${id}/comments/${commentId}`),
   listFeedbackVotes: (id: string) => api.get<FeedbackVote[]>(`/issues/${id}/feedback-votes`),
+  getCostSummary: (id: string) => api.get<IssueCostSummary>(`/issues/${id}/cost-summary`),
   listFeedbackTraces: (id: string, filters?: Record<string, string | boolean | undefined>) => {
     const params = new URLSearchParams();
     for (const [key, value] of Object.entries(filters ?? {})) {