stat-cache-hitrate: support --tail size/time shorthand and remove --watch mode

[mouxinqq]

Motivation

• Make the --tail CLI more flexible and user-friendly by supporting shorthand row counts (2k) and time windows (30m/2h/1d) while normalizing behavior across small/large files.
• Simplify the skill by removing the deprecated continuous --watch/monitoring mode and associated UI hints and templates.

Description

• Update documentation in SKILL.md to revise the analysis mode options and examples to document 2k row shorthand and 30m/2h/1d time windows, and remove references to the continuous --watch mode.
• Update references/report_templates.md to adjust the --tail quick-view template and remove --watch monitoring guidance.
• Implement parse_tail_arg in scripts/stat_cache_hitrate.py to accept numeric rows, Nk shorthand, and Nm/Nh/Nd time windows (returned as minutes like "<minutes>m"), add math import, and validate inputs with clear error messages.
• Normalize read_lines/extract_data to treat tail when it is explicitly provided (including time-window tails), thread the parsed tail variable through processing, and remove the --watch CLI option and its output message.

Testing

• No automated tests were added or run against these changes.

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Codex Task