[codex] fix project update date labels

[redreceipt]

Summary

• Render projects ending today as today in the Slack project update digest.
• Exclude projects starting today from the Projects Starting Soon Slack section.
• Add regression coverage for both cases in the project update unit tests.

Root Cause

The digest used short weekday labels for every non-overdue project target date, including deadlines that are today. The starting-soon window also included day zero, so projects starting today were still shown as upcoming.

Validation

• python -m unittest tests.test_jobs.PostProjectUpdatesTest
• ruff check .
• python -m unittest discover -s tests -p 'test_*.py'

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Note

Low Risk
Small presentation and window logic in the scheduled Slack digest; covered by unit tests with no auth or data changes.

Overview
Adjusts the daily Projects Ending Soon / Projects Starting Soon Slack digest in post_project_updates so same-day dates read clearly and “starting soon” means upcoming days only.

Projects whose target date is today now show Today in the line label instead of the short weekday (e.g. Sun). Projects whose start date is today are no longer listed under Projects Starting Soon; that section only includes starts 1–3 days out.

Unit tests in PostProjectUpdatesTest cover an ending-today line and assert starting-today projects are omitted.

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