Add time-range selection and improve session reports and table rendering for stat-cache-hitrate

[mouxinqq]

Motivation

• Enable analysis for a specified time window (--start/--end) and make the skill's interactive mode present discrete selectable options.
• Improve session-level diagnostics by surfacing per-session timestamps and prefill URL counts to aid time-scoped investigation.
• Produce more readable Markdown tables with fixed-width column alignment so terminal and document viewers show columns cleanly.

Description

• Update SKILL.md to document --start/--end support, require AskUserQuestion discrete options for analysis mode, and adjust output descriptions for session detail fields.
• Enhance session_analysis.py to include first_ts, last_ts, and prefill_url_count in per-session rows.
• Replace Markdown table rendering in stat_cache_hitrate.py with a width-aware renderer and alignment support, and introduce _extract_seq_num, _summarize_id_type_ranges, and sequence labels produced by _seq_label.
• Add session sequencing, id_type range summarization, overall time-span calculation, rename/reshape session table columns to avg-hit and purl_cnt, and include session time ranges and improved mapping/links in the generated session_hit_details.md.

Testing

• No automated tests were run for these changes.

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Codex Task