Show who owns a story — render the assignees Facility already mirrors

The problem

Every story on the board is unowned as far as the interface is concerned. Nowhere in apps/web is assignees read — not on the row, not on the story header, not on the pipeline board.

The data has been there all along, and #70 just made it meaningful:

• gh_issues.assignees is mirrored on every webhook and every sync (packages/db/src/schema.ts:234, services/api/src/github/issues-sync.ts:281);
• it is assembled into the story (services/api/src/pipeline.ts:178);
• it is serialised on both responses the web already fetches — PipelineStorySchema.assignees (services/api/src/routes/v1/github.ts:189) and StoryDetailSchema.assignees (:228);
• and since #70, triggering an agent assigns the triggering user on the issue and on the PR the run later opens (services/api/src/routes/v1/github.ts:944, services/api/src/sandbox/orchestrator.ts:891).

So Facility writes ownership to GitHub, mirrors it back, ships it over the wire — and drops it at the last inch. On a board where several people dispatch agents against the same repository, "who is on this?" is unanswerable without opening GitHub.

What I propose

Render the assignee on the story row and in the story header, in the existing grammar: @login, mono, --dim, beside the labels and the timestamp in IssueRow (apps/web/components/issues/issue-row.tsx:84-92). Several assignees: the first plus +N. Unassigned: render nothing, not a placeholder — the rows are dense and unassigned is the common case in Backlog.

Two things that make it more than decoration, worth the same slice:

• Avatars. users.avatar_url exists for Facility users, but a GitHub assignee need not be one; https://github.com/{login}.png?size=40 needs no new column and no new sync. Where a deployment must not let the browser reach github.com, the initial-letter fallback the topbar already uses is the answer.
• A "mine" filter on the Stories board, next to the stage chips, matching against me.principal.githubLogin (added by #70, services/api/src/routes/v1/shared.ts:123). That is what turns a rendered field into a working view.

What I considered instead

• Showing the last run's triggeredBy as the owner. That is who dispatched the most recent agent — often, but not always, who owns the story — and it is empty for anything nobody has dispatched yet. The GitHub assignee is the answer the rest of the world already agrees on.
• A Facility-native owner field. A second source of truth for something GitHub models, with no writer, no sync path and no reconciliation story.

The change is apps/web-only: the data is already on the wire, and apps/web/test/pipeline-story.test.ts is where the row's rendering rules are pinned.

Related: #70, #18. Findings are against 5712bf8.

Start with apps/web/components/issues/issue-row.tsx:84-92 and apps/web/test/pipeline-story.test.ts, then trace the story header and Stories board stage-chip components. Confirm the existing assignees data is rendered with the specified unassigned and multiple-assignee rules, avatar fallback behavior, and mine filter, with tests covering the rendering rules.