Add --weekday and --day-summary options to rssjournal

[adulau]

Motivation

• Provide an option to display the weekday name next to generated YYYY-MM-DD dates in daily page titles and yearly index listings.
• Provide a one-line autogenerated summary describing the day's RSS activity to be inserted/updated at the top of each daily journal page.

Description

• Add two CLI flags --weekday and --day-summary and wire them through write_journal and CLI parsing to control label formatting and summary insertion.
• Implement format_day_label and use it to produce title labels like 2026-06-06 (Saturday) and update yearly index labels accordingly.
• Add build_day_summary and upsert_day_summary (with DAILY_SUMMARY_PREFIX) to generate and insert/update a concise one-line summary based on item count, top sources, and representative titles.
• Update front-matter handling with ensure_markdown_front_matter(update_title=True) and update_front_matter_title to keep page titles in sync when weekday labels are enabled, and update README.md to document the new options.

Testing

• Ran python3 -m py_compile bin/rssjournal.py which succeeded.
• Ran git diff --check and lint-style checks which produced no issues.
• Performed a functional smoke test using stubbed feedparser/bs4 modules exercising --weekday --day-summary, which produced correctly-formatted daily and yearly pages.
• Attempted python3 -m pip install --user -r REQUIREMENTS but it failed due to package index access being blocked (403 tunnel error), so dependency installation could not be fully validated.

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Codex Task