Add session-level analysis and window-merging to stat-cache-hitrate skill

[mouxinqq]

Motivation

• Provide deeper session-dimension diagnostics (per-session hit rates, worker switches and sharp-drop requests) to help investigate cache-aware scheduling behavior.
• Reduce noise in per_window_data.md by compressing long runs of empty windows so reports are easier to read.
• Update skill docs and report templates to advertise the new outputs and clarify log path / default options.

Description

• Added scripts/session_analysis.py implementing compute_session_details and summarize_session_details to produce per-session rows and summary metrics.
• Added scripts/window_utils.py with merge_blank_window_rows to compress consecutive empty 5s windows into a 3-line summary block.
• Integrated the new modules into scripts/stat_cache_hitrate.py to merge window rows, emit details/session_hit_details.md, and add references/summary to the main report; adjusted imports and report text accordingly.
• Updated SKILL.md to refine the user prompt for log path defaults/options and report_templates.md to include the session_hit_details file and describe merged window behavior.

Testing

• No automated tests were added or executed as part of this change.

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Codex Task