feat(agent-manager): add fork action to completed sessions

[marius-kilocode]

Completed Agent Manager sessions only expose session forking through the tab context menu, which keeps a common follow-up action hidden even though the completed-session action row has room.

This adds a visible Fork Session action for writable, idle Agent Manager sessions with messages. It reuses the existing worktree-aware fork flow, remains absent from the narrower sidebar, and uses the Git-fork icon consistently with inline message and tab context-menu fork actions.

[kilo-code-bot]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

The implementation is clean and well-scoped. A few notes worth calling out:

• canFork status check is intentionally stricter than idle(): idle() (line 52) returns true for retry and offline states too (status !== "busy"), while canFork (line 191) uses session.status() === "idle" — this correctly prevents the fork button from appearing during transient retry/offline states.
• Handler reuse is safe: handleForkSession(sessionId, messageId?) in AgentManagerApp.tsx is used for both onForkMessage and onForkSession. The optional messageId makes it compatible for both call sites without issue.
• WorktreeModeProvider in the story: correctly simulates a non-sidebar (Agent Manager) context so isSidebar() returns false, making the fork button visible in the story.
• Icon fix in sortable-tab.tsx: changing the context-menu fork item icon from branch to fork is a legitimate consistency fix, bringing it in line with the new action button and inline message fork.

Files Reviewed (5 files)

• .changeset/fork-completed-agent-sessions.md
• packages/kilo-docs/public/img/screenshot-tests/kilo-vscode/visual-regression/chat/chat-view-agent-manager-completed-chromium-linux.png
• packages/kilo-vscode/webview-ui/agent-manager/AgentManagerApp.tsx
• packages/kilo-vscode/webview-ui/agent-manager/sortable-tab.tsx
• packages/kilo-vscode/webview-ui/src/components/chat/ChatView.tsx
• packages/kilo-vscode/webview-ui/src/stories/chat.stories.tsx

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_{Reviewed by claude-4.6-sonnet-20260217 · 1,329,184 tokens}

_{Review guidance: REVIEW.md from base branch main}