Add chat history gantt timeline viewer

[aniket-s-kulkarni]

Summary

• add a command and a path for visualizing tool execution timelines
• add a Textual TUI with step grouping, frozen legend/time axis, think-time visualization, and duration-based coloring
• add tests covering dump parsing, step grouping, think-time insertion, and CLI entrypoints

Testing

• uv run pytest tests/test_commands/test_chat.py tests/test_cli.py

[chatgpt-codex-connector]

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