docs(command-zone): add developer and user documentation

Add comprehensive documentation for the command zone feature.

Developer documentation:
- command_zone_architecture.md: detailed architecture overview
  - Layer separation (types, state, logic, graphics)
  - Component interactions and dependencies
  - Testing strategy and coverage
- Index update to include new architecture doc

User documentation:
- commander_format.md: how to use Commander format in Cockatrice
  - Zone configuration options
  - Gameplay mechanics (commander tax, zone reveals)
  - Tips for Commander games
- Index update to include new format doc

doc/doxygen/extra-pages/developer_documentation/command_zone_architecture.md

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+@page command_zone_architecture Command Zone Architecture
+
+The command zone feature implements Commander format support in Cockatrice,
+providing zones for commander cards with manual tax tracking.
+
+# Architecture Overview
+
+The command zone follows Cockatrice's logic/graphics separation pattern:
+
+```
+┌───────────────────────────────────────────────────┐
+│ Graphics: CommandZone, ZoneToggleButton,          │
+│           CommanderTaxCounter                     │
+└───────────────────────────────────────────────────┘
+                    │ signals/slots
+                    ▼
+┌───────────────────────────────────────────────────┐
+│ Logic: CommandZoneLogic (card storage, positions) │
+└───────────────────────────────────────────────────┘
+```
+
+# Zone Types
+
+| Zone | Name Constant | Purpose | Enablement |
+|------|---------------|---------|------------|
+| Primary | `ZoneNames::COMMAND` | Main commander, always visible | `enable_command_zone` |
+| Partner | `ZoneNames::PARTNER` | Partner commander, collapsible | `enable_command_zone` |
+| Companion | `ZoneNames::COMPANION` | Ikoria companion card | `enable_companion_zone` |
+| Background | `ZoneNames::BACKGROUND` | Baldur's Gate background | `enable_background_zone` |
+
+All zones use the same `CommandZone` and `CommandZoneLogic` classes,
+differentiated by the `isPrimaryZone` flag. Primary and Partner zones are always
+created together when command zone is enabled. Companion and Background zones
+can be enabled independently via separate game creation options.
+
+Primary and Partner zones hold a single card (commander rules, tax counter positioning); Companion and Background support multiple cards for edge cases. See `supportsMultipleCards()` in `command_zone_types.h`.
+
+# Key Components
+
+## CommandZoneLogic
+Manages the card list for a command zone. Overrides `addCardImpl()` to support
+positional insertion when cards are reordered via drag-and-drop.
+
+@see CommandZoneLogic
+
+## CommandZone
+Graphics layer handling rendering and input. Features:
+- Vertical card centering
+- Expand/collapse for partner zone
+- Minimize mode (25% height)
+- Drag-and-drop support
+
+@see CommandZone
+
+## CommanderTaxCounter
+Visual counter showing cumulative commander tax. Positioned at top-left of
+the zone. Players manually increment the counter when casting their commander.
+
+@see CommanderTaxCounter
+
+## ZoneToggleButton
+Small button at the seam between zones. Clicking toggles the sibling zone
+visibility. Used by the primary command zone to toggle the partner zone, and
+can be reused by any zone that needs to toggle a sibling.
+
+@see ZoneToggleButton
+
+# Common Modifications
+
+## Adding a new zone property
+1. Add the property to `CommandZoneLogic`
+2. Emit a signal when the property changes
+3. Connect the signal in `CommandZone` to update visuals
+
+## Changing zone appearance
+1. Modify `CommandZone::paint()` for background/border
+2. Modify `reorganizeCards()` for card layout
+3. Update `boundingRect()` if size changes
+
+## Modifying tax behavior
+The tax counter value is managed by the server. To change tax logic:
+1. Server-side changes in `libcockatrice_network/libcockatrice/network/server/remote/game/server_player.cpp`
+2. Counter display changes in `CommanderTaxCounter`
+
+# Menu Integration
+
+The command zone context menus are managed by `PlayerMenu`:
+
+| Menu | Zone | In Player Menu | Special Features |
+|------|------|----------------|------------------|
+| commandZoneMenu | Primary | Yes | View partner zone, partner toggle, tax counters for both |
+| partnerZoneMenu | Partner | No (zone right-click only) | Self-toggle, partner tax counter |
+| companionZoneMenu | Companion | Yes | Standard zone actions, no tax counter |
+| backgroundZoneMenu | Background | Yes | Standard zone actions, no tax counter |
+
+The `commandZoneMenu` consolidates all commander-related actions including partner
+zone access ("View partner zone" action). The `partnerZoneMenu` is not added to
+the main player menu but remains attached to the partner zone itself for direct
+right-click access. This consolidation reduces menu clutter while maintaining
+full functionality.
+
+All menus are created in the `PlayerMenu` constructor and attached to their zones
+in `setMenusForGraphicItems()`. Companion and Background zones have simpler
+menus without tax counter adjustments (these mechanics don't use commander tax).
+
+@see CommandZoneMenu
+@see PlayerMenu