feat(ui): add dedicated Agent subagent rendering with nested tool call display

Render Agent/Explore/Plan tool calls in a visually distinct collapsible
container with colored left border, replacing the generic tool card. Parse
subagent JSONL transcripts from {sessionId}/subagents/ to extract and
display the actual tool calls (Bash, Read, Grep, etc.) the subagent
executed, reusing the existing ToolCallPart for consistent appearance.

- Add AgentToolCallPart component with collapsible body, prompt section,
  execution stats, and nested tool call list via render prop injection
- Add AgentExecutionStats and AgentToolCall types (Rust + TypeScript)
- Parse toolUseResult.agentId to locate and read subagent JSONL files
- Validate agentId against path traversal before filesystem access
- Pass agentStats through adapter for both ID-matched and positional
  tool result pairing
- Strip agentStats in nested render to prevent recursive Agent expansion
- Add i18n keys for agent UI labels across all 10 languages

src/i18n/messages/en.json

@@ -1703,6 +1703,13 @@
         "subjectLabel": "Subject",
         "taskLabel": "Task",
         "nameLabel": "Name:",
+        "agentPromptLabel": "Prompt",
+        "agentModelLabel": "Model",
+        "agentStatsBash": "Commands",
+        "agentStatsRead": "Files read",
+        "agentStatsSearch": "Searches",
+        "agentStatsEdit": "Edits",
+        "agentStatsOther": "Other",
         "field": {
           "file": "File",
           "notebook": "Notebook",