chore: remove small dead leftovers - #14293

Removes three pieces of dead code found during a cleanup pass. The diff is net negative (14 insertions, 52 deletions).

Changes

• Mermaid default labels: packages/ui/src/kilocode/mermaid/markdown-mermaid.ts had a hardcoded English labels object and a mergeLabels fallback. The only caller (packages/ui/src/components/markdown.tsx) always builds a complete MermaidLabels from i18n keys, so every default was shadowed. Removed the default and mergeLabels; renderMermaid now takes the caller's labels directly. renderMermaid is not exported from any public entry.
• Duplicate Mermaid story: packages/ui/src/kilocode/mermaid/markdown-mermaid.stories.tsx exported a Mermaid story that duplicates the MarkdownMermaid story in packages/kilo-vscode/webview-ui/src/stories/shared.stories.tsx, which the same Storybook loads. Removed only the duplicate export; MermaidError and MermaidThemes remain.
• Rename-only alias: packages/kilo-vscode/webview-ui/src/components/chat/background-agents.ts exported children as a pass-through to the private taskChildren. Exported taskChildren directly and updated the importers (ChatView.tsx, SubagentPanel.tsx, and the unit test) rather than keeping an alias.

No behavior change. No new helpers or tests.

Evidence

• bun run --cwd packages/kilo-vscode lint passes.
• bun test tests/unit/background-agents.test.ts from packages/kilo-vscode: 24 pass.
• bun test src/kilocode/mermaid from packages/ui: mermaid tests pass. The markdown-bidi.test.ts case fails on a pre-existing unbuilt @opencode-ai/core module, unrelated to this change.
• bun run --cwd packages/kilo-vscode typecheck and packages/ui typecheck report only pre-existing environment errors (unbuilt workspace packages and stale SDK in node_modules), none in touched files.