Timeline: add Dependabot assignment and Copilot work timeline events

[janmaarten-a11y]

Summary

This is a Phase 2 follow-up (github/primer#6663) that fills the remaining gaps in the Dependabot alert Timeline event stories. It adds the three live Dependabot timeline events that our merged stories were missing: assignment, copilot_work_started, and copilot_work_finished. The work is Storybook-only and stays surgical: it adds two new exported stories to the existing Timeline.dependabot.features.stories.tsx file and reuses the shared internal story helpers.

What it adds

• EventAssignment: mirrors the existing Secret scanning assignment story with five variant sections (self-assigned, assigned another user, removed own assignment, unassigned another user, and a combined assign + unassign). It uses a default (gray) PersonIcon badge and renders both the actor and the assignee with avatars, following this file's own linked, muted UserActor convention.
• EventCopilotWork: one story with two variant sections (work started and work finished). The started row uses a default GitBranchIcon badge and a right-aligned small anchor Button ("View session"); the finished row uses a default RepoPushIcon badge. In both rows "Copilot" renders as bold plain text (a <strong>), matching how this file already renders bold non-link text.

Source of truth

Dependabot is server-rendered ERB (ViewComponent), so each event is translated faithfully from the live monolith components in app/components/dependabot_alerts/timeline_items/:

• assignment_component.{rb,html.erb} (badge person; action_text collapses self-actions to "self-assigned this" / "removed their assignment", otherwise the bare action word plus the assignee).
• copilot_work_started_component.{rb,html.erb} (badge git-branch; optional right-floated "View session" anchor button when a session URL is present).
• copilot_work_finished_component.{rb,html.erb} (badge repo-push).

Each new story carries a JSDoc doc-comment citing its ViewComponent source, matching the style of the existing stories.

Out of scope

Per the Phase 2 plan, these stories are intentionally not wired into Timeline.docs.json or build.ts, and no data-* filtering attributes are added (deferred to Phase 3). No new CSS was needed.

Testing & Reviewing

• prettier, eslint, stylelint, and type-check all pass with zero new errors.
• Rendered EventAssignment and EventCopilotWork headlessly in Chromium against the local Storybook build and confirmed: assignment rows show actor and assignee avatars with the correct copy; Copilot rows show bold plain "Copilot" (no avatar), the git-branch and repo-push badges, and the "View session" button only on the started row, all with zero component or React console errors or warnings.

Note: primer/react main is currently locked for a release, so this will not appear in the public Storybook until the release cuts.

Rollout strategy

• None; Storybook-only stories that do not change any published, consumer-facing API. A skip changeset label is applied.

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[Copilot]

Pull request overview

This PR extends the Dependabot Timeline Storybook feature stories to cover remaining live event types from the server-rendered Dependabot alert timeline, specifically focusing on assignment and Copilot work lifecycle events.

Changes:

• Added a new EventAssignment story with five variants covering self-assignment, assignment/unassignment of another user, and a combined case.
• Added a new EventCopilotWork story with “work started” and “work finished” variants, including a right-aligned “View session” action for the started state.
• Introduced two additional public avatar fixtures for use in the new assignment examples.

Show a summary per file

File	Description
packages/react/src/Timeline/Timeline.dependabot.features.stories.tsx	Adds new Dependabot timeline event stories for assignment and Copilot work started/finished, aligning Storybook coverage with live ERB behavior.

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 1
• Review effort level: Low