Archive app-navigation and planner-multiplayer-awareness, sync specs

Both changes fully implemented. Sync delta specs to main:
- map-display: add Planner home page nav and cursor rendering requirements
- planner-session: add session participant awareness requirement

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

openspec/changes/app-navigation/specs/map-display/spec.md

@@ -1,12 +0,0 @@
-## MODIFIED Requirements
-
-### Requirement: Planner home page
-The Planner home page SHALL provide a clear call-to-action to create a new planning session and a link back to the home page from within sessions.
-
-#### Scenario: Home page CTA
-- **WHEN** a user visits the Planner home page
-- **THEN** a prominent "Start Planning" button is visible that links to `/new`
-
-#### Scenario: Session home link
-- **WHEN** a user is in a planning session
-- **THEN** the header contains a link back to the Planner home page