feat: add bounded agenda view

Add a read-only 42-day occurrence agenda with keyboard navigation, six-week paging, exact return-to-grid focus, search and calendar overlays, and bounded recurrence expansion.

Keep agenda state outside persistence, document month-only mutations, and cover empty windows, filtering, overlays, navigation, rendering, and terminal restoration.

docs/ARCHITECTURE.md

@@ -42,10 +42,18 @@ rule cannot turn an interactive query into unbounded work.
 
 Calendar visibility is session presentation state. The controller derives its
 calendar list from event calendar identifiers and applies the same domain
-predicate to month rendering, focus traversal, and search. Hidden events remain
-in the canonical collection passed to persistence, and focus identities are
-always derived from that complete collection so filtering cannot renumber
-events with duplicate or missing UIDs.
+predicate to month rendering, agenda queries, focus traversal, and search.
+Hidden events remain in the canonical collection passed to persistence, and
+focus identities are always derived from that complete collection so filtering
+cannot renumber events with duplicate or missing UIDs.
+
+The agenda is a TUI projection over domain occurrence queries, not a second
+calendar model. It requests exactly one 42-day window at a time so recurrence
+expansion remains finite, preserves occurrence identity when returning to the
+month grid, and keeps its window and selection as non-persistent controller
+state. Calendar and search overlays may be opened above it, but mutations stay
+in the month workflow and continue to operate on the canonical event
+collection.
 
 Timed events retain their source basis: UTC, floating process-local time, or an
 IANA `TZID`. C++20 time-zone conversion keeps zoned recurrences at their civil