feat: add --group/-g flag to launch TUI scoped to a specific group

[johnuopini]

Summary

Add --group / -g CLI flag to launch the TUI locked to a specific group, showing only that group's sessions and children. Intended for automation workflows that open dedicated windows per group.

• --group <name> filters the session list, group tree, and all hotkey operations to the specified group and its children
• Validates group existence at startup (exits with code 2 if not found)
• Scopes hotkeys: n/N create sessions under the scope root, g creates subgroups under the scope, M only shows in-scope move targets, d prevents deleting the scope root (with feedback)
• Shows scope indicator in the title bar and a dedicated empty state when the group has no sessions
• Adds startup flags section to help overlay
• Composes with the existing status filter (!, @, etc.)

Usage

agent-deck --group work        # scoped TUI
agent-deck -g work             # short form
agent-deck --profile dev -g work  # combined with profile

Test plan

• TestExtractGroupFlag covers all flag forms (short, long, equals, space-separated, combined with -p)
• TestSetGroupScope and TestGroupScopeNormalization verify field setting and normalization
• TestRebuildFlatItemsGroupScope verifies filtering excludes out-of-scope groups
• TestRebuildFlatItemsGroupScopeComposesWithStatusFilter verifies scope + status filter compose correctly
• go vet clean, all tests pass on cmd/agent-deck and internal/ui packages