[codex] fix journalProjects auxiliary file filtering

[wangping88]

Summary

• Reuse the shared journal file filter when listAllProjects() scans project journals.
• Keep the existing date-prefix requirement for project listings so freeform markdown files do not become project entries.
• Add regression coverage for journalProjects() to ignore merged entries, capture notes, weekly rollups, index files, and non-date markdown.

Why

journalProjects() depends on listAllProjects(), which had a local journal filename filter that drifted from the shared isJournalFile() helper. Auxiliary files such as *.merged.md could be counted as real journal entries and make project lists report the wrong latest entry or entry count.

Tests

• npm run build
• npm run lint
• npm test