Add list_assessments MCP tool - #610

Summary

• New read-only tool list_assessments: one list of the user's assessments in a date range (default today to 30 days ahead) with an optional subject. It merges the exams with agenda entries that announce a test, presentation or hand-in (German, French, Italian and English keywords such as Test, Prüfung, LK, Lernkontrolle, Vortrag, Präsentation, Abgabe, Projekt, mündlich, examen, épreuve, contrôle, verifica, esame, presentation, hand-in; a word after "kein", "ohne", "no" and the like does not count; agenda entries of type Test always count).
• Each item has date, time if known, subject, title, kind (exam, presentation, hand-in, other), source (exams or agenda) and the teacher where the school passes it on. The same test in exams and agenda for the same subject and day is listed once (the exam wins and takes the agenda's time), repeated agenda entries are collapsed.
• subject is matched through the shared subject name table, so Mathe, Mathematik and maths find the same entries. SubjectAliases.cs, SubjectTeachers.cs and SwissTime.cs are identical to the files in Add list_subjects MCP tool with subject aliases #608 (list_subjects), so the two PRs can merge in either order.
• Needs Exams read; the agenda part needs Calendar read and is skipped (and reported in skipped) without it.
• The description and a note in every answer state that schedule documents (Masterplan, Semesterplan, Prüfungsplan) are not read and still need search and read_file.
• Helply's prompt now starts with list_assessments and keeps list_exams and list_agenda as fallback.

Closes #604