stat-cache-hitrate: add session-level analysis and window merge compression

[mouxinqq]

Motivation

• Improve the cache hitrate skill to surface session-level insights (per-session hit rates, worker switches, sharp drops) for better diagnosis of cache-aware scheduling effects.
• Reduce noise in per_window_data.md by collapsing long runs of empty windows so reports are easier to scan.
• Update documentation and report templates to reference the new session detail outputs and clarify log path options.

Description

• Added scripts/session_analysis.py to compute per-session rows and a session-level summary (compute_session_details and summarize_session_details).
• Added scripts/window_utils.py to merge consecutive blank window rows into a 3-line compressed block via merge_blank_window_rows to reduce detail noise.
• Integrated the new modules into scripts/stat_cache_hitrate.py to: merge blank windows before writing details/per_window_data.md, generate details/session_hit_details.md with overview and detail table when session data exists, and print the session details path to the CLI output.
• Updated SKILL.md to refine log path selection options and default behavior and updated references/report_templates.md to include the session_hit_details.md reference and example notes.

Testing

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

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Codex Task