Make usage primary, status as alias

[murrayju]

Promotes usage to the primary name for the space-usage command (CLI and MCP tool), with status kept as an alias so existing usage continues to work.

Changes

• CLI: ghost status → ghost usage (with status as alias). Renamed internal/cmd/status.go → usage.go (plus test file) and buildStatusCmd/outputStatus → buildUsageCmd/outputUsage.
• MCP: ghost_status → ghost_usage. Renamed internal/mcp/status.go → usage.go and the associated input/output/handler types.
• Updated README.md, CLAUDE.md, and regenerated docs/cli/.
• Updated internal/cmd/mcp_list_test.go expected tool list.

[nathanjcochran]

The only bummer about this is that it makes the CLI and MCP commands no longer match up with the GET /spaces/{space_id}/status API endpoint. So far, we've been pretty good about keeping them 1:1. Maybe we should consider renaming the API endpoint too (or really, duplicating it and keeping the old deprecated endpoint for backwards-compatibility)?

[murrayju]

Maybe we should consider renaming the API endpoint too (or really, duplicating it and keeping the old deprecated endpoint for backwards-compatibility)

I'll do this next